A method was developed for selectively isolating genes from localized regions of the human genome that are contained in interspecific hybrid cells. Complementary human DNA was prepared from a human-rodent somatic cell hybrid that contained less than 1% human DNA, by using consensus 5' intron splice sequences as primers. These primers would select immature, unspliced messenger RNA (still retaining species-specific repeat sequences) as templates. Screening a derived complementary DNA library for human repeat sequences resulted in the isolation of human clones at the anticipated frequency with characteristics expected of exons of transcribed human genes--single copy sequences that hybridized to discrete bands on Northern (RNA) blots.