Zero In on B Cells with the Mb1-iCre Mouse

Q: What is the Cre-Lox system and how does it achieve tissue-specific knockout?

The Cre-Lox system consists of Cre recombinase and LoxP sites. By crossing a Cre mouse model line (driven by a tissue-specific promoter) with a Floxed mouse, the gene of interest is only excised in tissues where Cre is expressed. This allows for localized gene deletion and bypasses the potential embryonic lethality associated with global knockouts.

Q: Can Floxed Mice Change Based on Gender?

The "floxed" genetic modification itself is typically autosomal and stable across genders; however, the phenotypic outcomes after crossing with a Cre driver can vary. Hormonal and metabolic differences between male and female floxed mice may influence the efficiency of gene excision or the resulting biological response. It is critical to monitor sex-specific expression patterns, especially when using promoters influenced by sex hormones, to ensure experimental consistency and to account for any dimorphic physiological effects in the conditional knockout model.

Q: What Is Flox Flox Mice?

A Flox/Flox mouse (or floxed mouse) is a sophisticated genomic model used in preclinical research to study the function of specific genes within a living organism. The term "floxed" stands for "flanking LoxP," referring to a gene or DNA segment that has been "sandwiched" between two LoxP (locus of crossover in P1) recognition sites using Cre-LoxP recombinase technology.

Q: How to Genotype Cre Flox Mice?

Genotyping Cre-Flox mice is a critical step in preclinical research to confirm the presence of both the Cre recombinase transgene and the floxed alleles (LoxP-flanked DNA segments) in your experimental cohort. This process ensures that gene deletion occurs only in the targeted tissues at the intended time.

Cyagen Technical Content Team | September 02, 2025

Mb1-iCre Mouse Model

Innovative tool tailored for precise & highly-efficient B cell targeting.

Contents

01. Ophthalmic Gene Therapy CRO Service Platform 02. About InnoVec Bio 03. About Cyagen

Introduction to Mb1-iCre Mice for B Cell Research

In immunology, B cells are a crucial component of the adaptive immune system. To better understand their function, scientists have developed specialized research tools and models. Among these, the Mb1-Cre mouse has gained prominence for its high recombination efficiency in B cell-specific studies. Codon-improved Cre recombinase (iCre) is an improved version of Cre that's designed to be expressed more efficiently and reduce the likelihood of epigenetic silencing in mammals. Additionally, iCre is expressed at higher levels in mice than P1 bacteriophage Cre, which facilitates high rates of Cre-driven recombination.

Today, we introduce the Mb1-iCre Mouse Model (Product No: C001552), an innovative tool tailored for precise & highly-efficient B cell targeting.

Understanding B Cell Receptors (BCRs)

The B cell receptor (BCR) is a hallmark surface complex of B cells, critical for regulating their survival, adaptation, activation, development, and transformation. The BCR consists of a membrane-bound immunoglobulin (Ig) molecule non-covalently linked to the Igα (CD79a) and Igβ (CD79b) transmembrane signaling subunits. This complex plays a central role in antigen recognition and the adaptive immune response. When an antigen binds to the BCR, it triggers antigen-induced BCR signaling, activating a series of signal cascades and cellular processes.^[1]

The MB1 gene (also known as the CD79a gene) encodes the Igα protein–an integral component of the BCR complex–which is exclusively expressed in B cells beginning in the very early pro-B cell stage in the bone marrow.[3] The Igα protein is primarily expressed in lymphoid follicles and subpopulations of peripheral immune cells, with notable cytoplasmic expression characteristics.

Figure 1. Structure of the B cell receptor (BCR) complex.^[3]

Mb1-iCre Mouse Model: Features and Benefits

Cyagen has independently developed the Mb1-iCre Mouse Model (Product No: C001552) using advanced gene editing techniques. This model incorporates a codon-optimized iCre recombinase element with high recombination activity that was knocked into the endogenous Cd79a gene of mice, enabling its specific expression in B cells. This model generation strategy disrupts the expression of the mouse's endogenous Cd79a gene, ensuring highly efficient recombination activity in B cells.

When Mb1-iCre mice are bred with mice carrying loxP-flanked sequences, the offspring may undergo Cre-mediated recombination that occurs predominantly in B cells. Notably, the Cre recombinase activity is higher in the early stages of B cell development.

1. Cre recombinase activity in lymphoid B cells

To assess the efficiency of Cre recombination in B cells, Mb1-iCre mice were crossed with ROSA26-LSL-tdTomato mice, which express tdTomato fluorescent protein conditionally.

Figure 2. Expression of Cre recombinase in lymphoid B cells (mCD45+ mCD19+) in peripheral blood, spleen, and bone marrow.

Mating the Mb1-iCre mice with ROSA26-LSL-tdTomato mice yielded offspring from which peripheral blood, spleen, and bone marrow tissues were collected. TdTomato protein expression was detected by fluorescence-activated cell sorting (FACS). Flow cytometry (FACS) analysis revealed:

Peripheral Blood B Cells: Strong tdTomato expression in the peripheral blood lymphoid B cells of Cre+ mice (Mb1-iCre[KI/+];ROSA26-LSL-tdTomato[CKI/+]), with a recombination efficiency of over 98%.
Splenic Lymphoid B Cells: Recombination efficiency in was approximately 80%
Bone marrow lymphoid B cells: Recombination efficiency ranged from 76.93% to 80.66%.

2. Cre Recombination Activity in Lymphoid T Cells

Figure 3. Expression of Cre recombinase in lymphoid T cells (mCD45+ mCD3+) in peripheral blood, spleen, and bone marrow.

In contrast, minimal Cre activity was observed in T cells. FACS results showed that in Cre+ mice:

Peripheral Blood T Cells: 1.89%-2.21% tdTomato expression.
Splenic T Cells: 2.49%-3.72% tdTomato expression.
Bone Marrow T Cells: No significant tdTomato expression detected in CD3+ T cell populations.

Conclusion

The Mb1-iCre Mouse Model (Product No: C001552) demonstrates exceptional specificity and recombination efficiency in lymphoid B cells. It serves as an invaluable tool for tissue-specific research targeting lymphoid B cells, ensuring precise genetic modifications.

Alongside our Mb1-iCre mice, Cyagen offers a broad range of inducible Cre mouse models designed to target specific cell types and tissues, supporting diverse research needs across immunology, hematopoiesis, and metabolism. Below is a list capturing some of our newly available Cre, iCre & CreERT2 mouse models alongside their specific tissue/cell applications:

Product Number	Product	Tissue/Cell Expression
I001184	Cd19-Cre	B lymphoid lineage cells
I001160	Csf1r-IRES-iCre	Macrophages, dendritic cells, and bone marrow-derived granulocytes
I001171	Cd2-IRES-iCre	T cells, B cells
I001172	Cd68-P2A-iCre	Monocytes, macrophages
I001173	Klrd1-P2A-iCre	Bone marrow, natural killer (NK) cells
I001198	Prdm1-iCre (Blimp1-iCre)	Primordial germ cells
I001146	Cpa3-P2A-iCre-EGFP	Mast cells, basophils, certain T cell progenitor populations, thymic T cells, and hematopoietic progenitor cells
I001169	Adgre1-P2A-CreERT2	Macrophages
I001170	Cd2-P2A-CreERT2	T cells, B cells
I001205	Lrat-P2A-tdTomato-T2A-CreERT2	Hepatic stellate cells and other cells expressing the Lrat gene.
C001558	Agrp-IRES-CreERT2-P2A-tdTomato	Arcuate nucleus (ARC) region of the hypothalamus.

By providing models that target specific lineages with high precision, Cyagen continues to support advancements in preclinical research.

Comprehensive Cre Driver and KO/cKO Mouse Models

Cyagen is committed to expanding our library of pre-developed rodent models to serve as a comprehensive resource for researchers. We offer a robust lineup of specialized inducible & Cre driver mouse lines– including iCre, Dre, CreERT2, fluorescent reporter lines, Rosa26, and more – to support precise genetic modifications in various tissues.

In addition to our inducible Cre lines, the Cyagen’s Knockout Catalog Models Repository (eBank) provides a selection of knockout (KO) and conditional knockout (cKO) lines available for diverse research applications. Search Cyagen’s extensive selection of KO/cKO mouse models now for your gene(s) of interest: Knockout Catalog Models Repository (eBank).

Comprehensive Cre Driver and KO/cKO Mouse Models

Cyagen's drug screening and evaluation mouse model platform can support your R&D by providing comprehensive model services and preclinical evaluations: from development of conditional knockout (cKO) and conditional knockin (cKI) mouse models, through phenotype analysis and in vivo efficacy evaluations. Our one-stop research colony maintenance solutions are designed to accelerate your preclinical R&D and bridge the translational gap towards effective clinical outcomes.

For more information on Cyagen's innovative mouse models and CRO services, contact us for a complimentary project consultation.

References

[1] Tkachenko A, Kupcova K, Havranek O. B-Cell Receptor Signaling and Beyond: The Role of Igα (CD79a)/Igβ (CD79b) in Normal and Malignant B Cells. Int J Mol Sci. 2023 Dec 19;25(1):10. doi: 10.3390/ijms25010010.

[2] Aribi, M. (2020). Introductory Chapter: B-Cells. IntechOpen. doi: 10.5772/intechopen.90636

[3] Hobeika E, Thiemann S, Storch B, Jumaa H, Nielsen PJ, Pelanda R, Reth M. Testing gene function early in the B cell lineage in mb1-cre mice. Proc Natl Acad Sci U S A. 2006 Sep 12;103(37):13789-94. doi: 10.1073/pnas.0605944103.