Hmx1, also known as H6 family homeobox 1, is a homeobox-containing transcription factor. It plays a crucial role in regulating multiple aspects of craniofacial development, being widely expressed in the eye, peripheral ganglia, and branchial arches [1,2]. Mutations in Hmx1 are associated with an ocular defect called Oculo-auricular syndrome of Schorderet-Munier-Franceschetti (MIM #612109) [1,2]. Genetic models, such as zebrafish and mice, are valuable for studying Hmx1's functions.

In zebrafish, knockdown of hmx1 led to microphthalmia, delayed withdrawal of retinal progenitors from the cell cycle, and increased cell death in the retina and brain, suggesting its importance in retinogenesis [3]. hmx1 -mutant zebrafish also showed premature vertebral mineralization, with changes in the expression of genes related to the BMP signaling pathway [2]. In mice, Hmx1dm/dm embryos had marked defects in the geniculate (VII) ganglion, with a reduction in general somatosensory neurons likely due to increased cell death [4]. In addition, duplications involving the long-range Hmx1 enhancer are associated with human isolated bilateral concha-type microtia [5]. In Awassi sheep, a duplication in part of the ECR near Hmx1 is related to microtia [6].

In conclusion, Hmx1 is essential for craniofacial development, retinogenesis, and the development of somatosensory neurons in the geniculate ganglion. Research using gene-knockout models in zebrafish and mice has revealed its role in various biological processes and its association with human and animal congenital defects such as oculo-auricular syndrome and microtia, providing insights into the underlying mechanisms of these diseases [1-5,9].