The FOS gene family consists of 4 members: FOS, FOSB, FOSL1, and FOSL2. These genes encode leucine zipperproteins that can dimerize with proteins of the JUN family (e.g., c-Jun, JunD), thereby forming thetranscription factor complex AP-1. As such, the FOS proteins have been implicated as regulators of cell proliferation, differentiation, and transformation. FosB and its truncated splice variants, ΔFosB and further truncated Δ2ΔFosB, are all involved in osteosclerosis, although Δ2ΔFosB lacks a knowntransactivation domain, in turn preventing it from affecting transcription through the AP-1 complex.