We intended to confirm genetically the involvement of the IDDMK1,2-22 gene in the pathogenesis of insulin-dependent diabetes mellitus (IDDM). For this purpose, we isolated a human endogenous retrovirus gene, possibly corresponding to IDDMK1,2-22. The isolated gene showed 99.8% and 99.7% homologies in nucleotide sequences to a part of the env region and of the 3'-LTR region, respectively, compared to those of IDDMK1,2-22 deposited in GenBank. The gene also showed a close relation to HERV-K18, of which the 3'-LTR sequence gave 99.5% homology. It seemed likely that these genes represented the same single gene. The newly isolated gene was present in the first intron of the CD48 gene and was located on chromosome 1q21.2-q22. A CA repeat marker was found approximately 20 kb upstream from the 5'-end of the 5'-LTR of the gene.