Tac4, encoding hemokinin-1 (HK-1) in mouse and rat, is a gene related to the tachykinin family. It has diverse functions, being involved in pain-inflammation modulation, immune regulation, respiratory and endocrine functions, as well as tumor genesis. HK-1 binds to the NK1 receptor, similar to Substance P, and plays roles in various biological processes [3,5].

In Tac4-/-mice, stress-induced mechanical hyperalgesia was decreased, suggesting HK-1's role in stress-induced pain [1]. Also, TAC4-/-mice had an increase of "fraction B" pro-B cells in the bone marrow, indicating an inhibitory role of HK-1 on developing B cells [4]. Additionally, Tac4-/-mice showed anxiolytic and anti-depressant-like actions through an unknown target-mediated mechanism [6]. In terms of bone, Tac4-/-mice had reduced mechanical bone strength, revealing its role in bone quality [2].

In conclusion, Tac4, through its product HK-1, plays important roles in pain regulation, hematopoiesis, mental behaviors, and bone quality. Studies using Tac4-/-mouse models have provided insights into its functions in these areas, which may have implications for understanding and treating related diseases such as stress-induced pain, immune-related disorders, mental illnesses, and bone diseases.