PHTF1 (putative homeodomain transcription factor 1), also known as PHTF, is a potential transcription regulator. It is a ubiquitously expressed integral, multipass membrane protein with predominant expression in testis. PHTF1 is associated with the ER (endoplasmic reticulum) and contains one bHLH (basic helix-loop-helix) domain. It is present in the cell during meiosis and spermiogenesis but, by the end of spermiogenesis, is released from the mature cell within the residual bodies. This implies that PHTF1 may play a role in the spermatozoa maturation process. In addition, PHTF1 is believed to interact with FEM1B and may be responsible for recruiting FEM1B to the surface of the ER membrane. This suggests that PHTF1 acts as a sequestering or anchoring protein for FEM1B. Two PHTF1 isoforms exist due to alternate splicing events. Isoform 2 is the shorter form and lacks the amino acid residues 648 to 762.