Transcriptional regulator that binds the cAMP response element (CRE), a sequence present in many viral and cellular promoters. Isoforms are either transcriptional activators or repressors. Isoform Tau is a transcriptional activator. Plays a role in spermatogenesis and is involved in spermatid maturation (By similarity). Binds DNA as a dimer. Interacts with FHL5. Interacts with CDC34. May interact with TSSK4. Additional isoforms seem to exist. Stimulated by phosphorylation. Phosphorylated on Ser-118 by TSSK4 in vitro. Belongs to the bZIP family.