The extent of reassociation and the thermal stability of DNA strands derived from different species are a measure of the evolutionary relation between species (1, 2). These measurements correlate closely with phylogenetic relationships developed on the basis of anatomical considerations and the fossil record (3-6). In this manuscript we use nucleic acid hybridization to examine the relatedness among primates of a particular set of cellular genes-those coding for the production of type C viruses. These viral genes are transmitted from parent to progeny along with other cellular genes and are postulated to be present in all tissues of each animal of a species (7). The type C virogene sequences offer several distinct advantages for the study of evolutionary relationships. The production of endogenous type C viruses, and the propagation of these viruses in heterologous hosts (8-10), enable the convenient isolation of a discrete set of cellular genes with speciesspecific properties. Single-stranded [3H]DNA transcripts of the viral RNA sequences can be used to detect information in the cellular DNA of related species. As described, the sets of type C virogene sequences appear to evolve faster than the unique sequence cellular genes. It is thus possible to establish taxonomic relationships between closely related species that have not been revealed previously by annealing of entire unique sequence DNA. 650 in reaction mixtures containing 10 mM Tris HCl (pH 7.4), 750 mM NaCl, 2 mM EDTA, 0.05% (w/v) Na dodecyl sulfate (NaDodSO4), 10,000-20,000 cpm/ml of [3H]DNA, and 1-3 mg of cellular DNA per ml (12). At various times, 0.05-ml portions were removed and frozen at -80°until digested with the single-strand-specific S1 nuclease (11,13). Cot values (Co is the concentration of cellular DNA in moles of nucleotide per liter, and t is the time in sec) were calculated as '/2(A2a/ml) X hr (14), and corrected to a monovalent cation concentration of 0.18 M (15). For determination of thermal melting curves on hydroxylapatite, 0.25-ml hybridization mixtures were diluted to 25 ml with 0.15 M sodium phosphate (pH 6.8) containing 0.1% NaDodSO4 and applied to 1.0 g of hydroxylapatite in waterjacketed columns at 55°. After the column was washed with 15 ml of this buffer at 550, the hybrids retained were eluted by increasing the column temperature at 50 intervals and washing with 15 ml of this buffer. The radioactive DNA eluting at each temperature was precipitated with calf thymus DNA carrier and trichloroacetic aoid and filtered. Abbreviations: NaDodSO4, sodium dodecyl sulfate; Tm, temperature at which 50% of the hybrids are dissociated.