Controls osteoclast survival and size (PubMed:18548006). As a dimer with JUN, activates LIF transcription (PubMed:18548006). Activates CEBPB transcription in PGE2-activated osteoblasts (By similarity). Heterodimer; with JUN (PubMed:18548006). Interacts with the BAF multiprotein chromatin-remodeling complex subunits SMARCB1 and SMARCD1 (PubMed:29272704). Interacts with ARID1A and JUN (PubMed:29272704). Mutant pups die within a week after birth. They exhibit severe osteopenia as early as 18.5 dpc, with a 50% decrease in mineralized bone. Osteoblast numbers are not altered. The number, the relative surface covered by osteoclasts and the mean osteoclast surface are increased 3-fold, 5-fold and 2-fold, respectively. Bone resorption activity is increased. This phenotype is due to hypoxia in long bones resulting from placental defects. Belongs to the bZIP family. Fos subfamily.