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. Plays a role in spermatogenesis and is involved in spermatid maturation (PubMed:10373550). May play a role in the regulation of the circadian clock: acts as a transcriptional repressor of the core circadian component PER1 by directly binding to cAMP response elements in its promoter. Binds DNA as a dimer. Interacts with FHL5 (By similarity). Interacts with CDC34 (PubMed:10373550). May interact with TSSK4 (By similarity). Expressed in testes (round spermatids) (at protein level). Isoform 14 is the major activator form in testes. Isoform 9 is ubiquitinated by CDC34 and RAD6B in order to be degraded by the proteasome. Stimulated by phosphorylation. Phosphorylated on Ser-116 by TSSK4 in vitro. Produced by alternative promoter usage. Produced by alternative splicing of isoform 1. Activator. Produced by alternative splicing of isoform 1. Activator. Produced by alternative splicing of isoform 1. Produced by alternative promoter usage. Activator. Produced by alternative promoter usage. Repressor. Produced by alternative splicing of isoform 6. Repressor. Produced by alternative splicing of isoform 6. Repressor. Produced by alternative splicing of isoform 6. Repressor. Produced by alternative promoter usage. Activator. Produced by alternative splicing of isoform 10. Repressor. Produced by alternative splicing of isoform 5. Repressor. Produced by alternative splicing of isoform 1. Activator. Produced by alternative splicing of isoform 1. Activator. Produced by alternative splicing of isoform 1. Repressor. Produced by alternative splicing of isoform 1. Repressor. Produced by alternative splicing of isoform 1. Repressor. Produced by alternative splicing of isoform 1. Repressor. Produced by alternative splicing of isoform 10. Activator. Produced by alternative splicing. Produced by alternative splicing. Produced by alternative splicing. Produced by alternative splicing. Produced by alternative splicing. Produced by alternative splicing. Belongs to the bZIP family. Truncated C-terminus.