Spermatogenic leucine zipper protein 1 is a member of the basic helix-loop-helix (bHLH) family of transcription factors which is expressed specifically in the testis and epididymis. Studies show that SPZ1 acts downstream of mitogen-activated protein kinase (MAPK), an extracellular signal-regulated kinase 1/2 to up-regulate cell proliferation and tumorigenesis. Also, through an interaction with proliferating cell nuclear antigen (PCNA) promoter, SPZ1 induced cell proliferation concomitant with an increase in PCNA gene expression.