Transcription factor that binds to the DNA sequence 5'-CANNTG-3'(E box) and the G-box motif. Directly binds to a guanine-rich region of the PCNA promoter and up-regulates its expression which in turn induces cell transformation and tumor formation. May play an important role in the regulation of cell proliferation and differentiation during spermatogenesis. Interacts with PPP1CC isoform gamma-2. This interaction can prevent SPZ1 binding to the E-box and inhibits PPP1CC activity. Expressed specifically in the testis and epidydimis. In the testis expressed in both germ cells and somatic cells (Sertoli and Leydig cells). Expressed in several tumor cell lines. First detected in the testis at postnatal day 10 and levels increase further and reach highest levels at day 40 and then gradually decrease. Down-regulated by testosterone and retinoic acid. Phosphorylated by MAPK1/ERK2 and MAPK3/ERK1. The helix-loop-helix and basic motifs form a Spz1 specific bHLH different from the classical one.