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 2, isoform 3 and isoform 4 are repressors, while isoform 1 is an activator. Plays a role in spermatogenesis and is involved in spermatid maturation. Binding of isoform 1 (activator) to CRE is increased by CREB3L4. The CREM isoform 1-CREB3L4 heterodimer functions through CRE and may recruit HIRA to CRE to regulate histone exchange (PubMed:16595651). Plays 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 (By similarity). Interacts with CDC34 (By similarity). Interacts with FHL5 (PubMed:10086359). May interact with TSSK4 (PubMed:26940607). Isoform 1 forms a heterodimer with CREB3L4 (PubMed:16595651). Expressed in the testis. In premeiotic germ cells, expressed at low amounts in the antagonist form. Subsequently, during spermatogenesis, isoform 1 (activator) is generated exclusively and in extremely high amounts. Isoform 11 is expressed in a circadian manner in the adrenal gland with an expression peak at ZT20 (at protein level). It is induced by cAMP in an immediate-early fashion and can repress its own production via a negative autoregulatory mechanism. Stimulated by phosphorylation (By similarity). Phosphorylated on Ser-116 by TSSK4 in vitro. Ubiquitinated by CDC34 and RAD6B in order to be degraded by the proteasome. CREM-null deficient mice display male infertility: while spermatogonia differentiate normally into round spermatids, all the elongating spermatids are eliminated by apoptosis during spermiogenesis. Produced by alternative promoter usage. Belongs to the bZIP family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.