This mixture of random hexanucleotides is used to prime DNA synthesis in vitro along multiple sites of denatured template DNA (1). This primer-template complex is a suitable substrate for DNA Polymerase I (Klenow Fragment). The newly synthesized complementary DNA is “oligo-labelled” by substituting any radiolabelled nucleotide for the appropriate nonradioactive nucleotide in the reaction mixture. Use of synthetic d(N)6 primer ensures the presence of virtually all sequence combination of hexamer primers which results in equally labelled DNA of high specific activity (1,2). Oligolabelling by this method generates probes which can be used to screen gene libraries (3), probe Southern and Northern blots (4,5), and for in situ hybridizations (6).