We isolated a novel gene encoding a protein highly homologous to human FK506-binding protein 12kDa (hFKBP-12) from a human fetal brain cDNA library and determined the full-length cDNA sequence. The cDNA clone contained the open reading frame of 324 nucleotides encoding 108 amino acid and revealed 76% identity in DNA sequence and 88% identity in predicted amino acid sequence with hFKBP-12. The DNA and amino-acid sequence of this gene, designated OTK4, also had homology with other FKBPs in species ranging from humans to prokaryotes. Recombinant protein, produced in E.coli transformed by a pGEX2T expression vector containing the OTK4 cDNA and purified, showed peptidyl-prolyl cis-trans isomerase activity like other FKBP proteins. An alternatively spliced form of the transcript found in the cDNA library contained a 45-bp insertion which included a stop codon. Although the biological function of the truncated version of OTK4 is unknown, both transcripts were ubiquitously expressed in human tissues examined by the reverse-transcriptase PCR (RT-PCR) method.