Putative transcription factor. Isoform 3 may play a role in early spermatogenesis. Efficient DNA binding requires dimerization with another bHLH protein. Isoform 3 is testis specific. Isoform 2 is pancreas specific. Isoform 3 is specifically expressed in primary spermatocytes at the pachytene stage, but not those at leptonema stage. Not expressed in other testicular cells, including spermatogonia located in the basal compartment of the seminiferous tubule or spermatids. Antibodies against TCFL5 are present in sera from patients with Chagas disease (also called American Trypanosomiasis), a disease caused by Trypanosoma cruzi. Two different epitopes that mimic Trypanosoma cruzi antigens have been identified: R1 and R3 epitopes, which are recognized by T- and B-cells, respectively.