Activator of cAMP-responsive element modulator (CREM) in testis (ACT) has recently been found in the mouse testis where it activates CREM, a transcription factor essential for the differentiation of spermatids into mature spermatozoa. The importance of CREM in human spermatogenesis prompted us to examine whether ACT was also present in the human testis. Western blot analysis, performed with an anti-mouse ACT serum, showed the presence of a single immunoreactive band of a size similar to murine ACT. A library screening resulted in the isolation and characterization of the complete cDNA which showed 88% homology with the mouse counterpart. The human ACT gene is composed of five coding exons, being the first untranslated, and the mRNA spans 835 nucleotides coding for a 284 amino acid protein. Expression studies by RT-PCR confirmed that ACT is present in normal human testis. The human ACT gene is localized on the chromosome 6.