FKB2 encodes a homolog of human FKBP-13, a membrane-associated binding protein for the immunosuppressants FK506 and rapamycin. FKB2 is located on the right arm of chromosome IV and contains an open reading frame of 135 amino acids, of which the first 17 residues comprise a putative hydrophobic leader peptide. Yeast FKBP-13 is homologous to human FKBP-13 (52% amino acid identity) and to FKBP-12, the major cytosolic receptor for FK506. In the alignment of FKBP-13 and FKBP-12 sequences, there are 28 invariant residues. Among these conserved residues are those that comprise the drug binding and peptidyl-prolyl cis-trans isomerase active site of FKBP-12. The phylogenetic conservation of the FKBP family suggest that the proteins are involved in a basic cellular function.