Chromosomal sites of integration of simian virus 40 DNA sequences mapped by in situ hybridization in two transformed hybrid cell lines

Simian virus 40 DNA sequences integrated on human chromosome 7 in two transformed human-mouse hybrid somatic cell lines, were mapped to a specific chromosomal locus by in situ hybridization. To detect the integrated viral DNA by in situ hybridization, we increased the sensitivity of the technique by using as a probe ¹²⁵I-labeled simian virus 40 cRNA (2 x 10⁹ to 3 x 10⁹ dpm/μg) prepared by in vitro transcription of simian virus 40 DNA with high-specific-radioactivity [¹²⁵I]CTP. Although the viral nucleic acid was shown by blot hybridization in the two cell lines to be integrated in different restriction fragments, it was shown by in situ hybridization in the two cell lines to map to the same position, q31, on the long arm of human chromosome 7. The viral DNA integration sites were localized with a precision of ±1 silver grain diameter, equivalent to about 6.2 x 10⁷ nucleotide pairs in the human genome. The procedures we describe may be adapted for localization of any gene on diploid chromosomes that can be cloned in a recombinant DNA vector.