A human genomic DNA fragment containing the 5' region of MCM4, a gene encoding replication protein MCM4/Cdc21 was isolated. At a distance of about 800 base pairs upstream of MCM4, the fragment was shown to also contain the 5' end of PRKDC, a gene encoding the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs). The genomic DNA fragment was used for hybridization to human metaphase chromosome spreads. The cytogenetic map location of the two closely adjacent genes was determined to be 8q12-->q13.