Several endocrine and neuronal functions are governed by the cAMP-dependent signalling pathway. In eukaryotes, transcriptional regulation upon stimulation of the adenylyl cyclase signalling pathway is mediated by a family of cAMP-responsive nuclear factors. This family consists of a large number of members that may act as activators or repressors. These factors contain the basic domain/ leucine zipper motifs and bind as dimers to cAMP-response elements (CRE). The function of CRE-binding proteins (CREBs) is modulated by phosphorylation by several kinases. Direct activation of gene expression by CREB requires phosphorylation by the cAMP-dependent protein kinase A to the serine-133 residue. Among the repressors, ICER (Inducible cAMP Early Repressor) deserves special mention. ICER is generated from an alternative CREM promoter and constitutes the only inducible cAMP-responsive element binding protein. Furthermore, ICER negatively autoregulates the alternative promoter, thus generating a feedback loop. In contrast to the other members of the CRE-binding protein family, ICER expression is tissue specific and developmentally regulated. The kinetics of ICER expression are characteristic of an early response gene. Our results indicate that CREM plays a key physiological and developmental role within the hypothalamic-pituitary-gonadal axis. We have previously shown that the transcriptional activator CREM is highly expressed in postmeiotic cells. Spermiogenesis is a complex process by which postmeiotic male germ cells differentiate into mature spermatozoa. This process involves remarkable structural and biochemical changes that are under the hormonal control of the hypothalamic-pituitary axis. We have addressed the specific role of CREM in spermiogenesis using CREM-mutant mice generated by homologous recombination. Analysis of the seminiferous epithelium from mutant male mice reveals that spermatogenesis stops at the first step of spermiogenesis. Late spermatids are completely absent, while there is a significant increase in apoptotic germ cells. A series of postmeiotic germ cell-specific genes are not expressed. Mutant male mice completely lack spermatozoa. This phenotype is reminiscent of cases of human infertility. We have shown that ICER is regulated in a circadian manner in the pineal gland, the site of the hormone melatonin production. This night-day oscillation is driven by the endogenous clock (located in the suprachiasmatic nucleus, SCN). The synthesis of melatonin is regulated by a rate-limiting enzyme, the serotonin N-acetyltransferase (NAT). By using the CREM-deficient mice and by analysis of the regulatory region of the gene encoding the serotonin NAT, we have established that ICER is responsible for the amplitude and rhythmicity of NAT and thus for the oscillation in the hormonal synthesis of melatonin.