From a human-leukocyte cDNA library, we cloned cDNA encoding a novel protein, which has a significant homology with the b subunit of ATP synthase (proton-transporting ATPase, F1F0-ATPase; EC220.127.116.11) derived from Anabaena sp. strain PCC 7120. The cDNA has an open reading frame of 1314 nucleotides corresponding to 438 amino acids. The coding sequence was 37.9% identical over 57 amino acid with b subunit of ATP synthase. The 34-amino-acid region of the predicted peptide sequence displays a coiled-coil motif that could form a complex with some other protein(s). We designated this novel gene as ATP-BL because of its homology to the b subunit of ATP synthase. The ATP-BL locus was mapped by fluorescence in situ hybridization (FISH) and radiation hybrid mapping to the q24 region of chromosome 16.