FosB is a member of the Fos (Finkel Osteogenic Sarcoma) gene family. It is the product of an immediate early gene that heterodimerizes with Jun family members in the AP-1 transcription complex. FosB has two splice forms that are 52-kDa and 36-kDa in size. Each form contains an N-terminal transcription-activating domain and a leucine-zipper region that allows for heterodimerization. The short form lacks a C-terminal transcription-activating domain that is necessary for gene activation.