The c-Myc proto-oncogene plays a significant role in cell proliferation, differentiation, transformation and apoptosis. A novel c-Myc binding protein, MYCBP (also designated AMY-1), binds to the transactivation domain of c-Myc and stimulates the activation of E-box-dependent transcription. MYCBP translocates from the cytoplasm to the nucleus during S phase when increased expression of c-Myc occurs. MYCBP and AAT-1 (AMY-1-associating protein expressed in testis 1) have been shown to associate with AKAP 149 and AKAP 84 in mitochondria of somatic cells and sperm, which suggests a role for MYCBP and AAT-1 in spermatogenesis. Expression of the AAT-1 gene is regulated by two different promoters, which result in various isoforms. One promoter generates expression of the AAT-1, AAT-1å, AAT-1∫ and AAT-1© isoforms, which are specifically expressed in testis, while the other promoter generates AAT-1L, AAT-1M and AAT-1S, which are differentially expressed.