LBP1 is a cellular protein that binds strongly to sequences around the human immunodeficiency virus-1 (HIV-1) initiation site and weakly over the TATA box.LBP1A and LBP1B are splice variants of UBP1, and LBP1C and LBP1D are splice variants of TFCP2. The deduced LBP1A and LBP1B proteins contain 504 and 541 amino acids, respectively. In vitro translation resulted in proteins with apparent molecular masses of 63 and 68 kD, respectively. LBP1A shares 72% amino acid identity with LBP1C, with higher identity in the N-terminal halves.LBP1A and LBP1B could activate transcription from a promoter containing an LBP1-binding site. Both isoforms could bind the TFCP2 isoform LBP1C and form heteromeric complexes. However, the TFCP2 isoform LBP1D inhibited binding of the other LBPs to the LBP1 recognition site on DNA.