TCFL5 [transcription factor-like 5 (basic helix-loop-helix)], also known as cha transcription factor, HPV-16 E2-binding protein 1, CHA, Figlb, E2BP-1 or MGC46135, is a 500 amino acid protein belonging to the basic helix-loop-helix (bHLH) family. TCFL5 localizes to the nucleus and may function as a transcription factor during early spermatogenesis. TCFL5 is known to form a complex with upstream stimulatory factor (USF)-1, thereby negatively regulating USF-dependent transcription. Six isoforms of TCFL5 exist due to alternative splicing; isoform 2 is pancreas specific and isoform 3 is testis specific showing expression in primary spermatocytes at the pachytene stage. Antibodies against TCFL5 have been identified in patients with Chagas' disease, which is caused by Trypanosoma cruzi infection.