Junctin is a calsequestrin binding protein detected in junctional sarcoplasmic reticulum of striated muscles. In the present study, the human cardiac junctin cDNA has been cloned by human heart cDNA library screening and RT-PCR, and the cDNA sequence has been determined. The deduced amino acid sequence of human junctin (210 aa) has 84% sequence identity to that of canine junctin identified previously. A human junctin isoform (isoform 1, 225 aa) was also identified and characterized. The isoform 1 has a 15 aa insertion at the amino acid residue 55 of the human junctin. Northern blot analysis revealed that the human junctin was present both in cardiac and skeletal muscles, and the sizes of the transcripts were approximately 3.0 and 4.2kb. Amino acid residues 6-78 of human junctin and 35-107 of human aspartyl beta-hydroxylase (hAspH) overlapped perfectly. The gene copy number of human junctin and hASPH was investigated by genomic Southern blot analysis using various restriction enzymes and a common DNA probe. The result showing a single hybridized DNA band at each restriction enzyme suggests that the same genomic region codes both junctin and hASPH.