It has been known for several years that deoxyribonucleic acid may normally contain 5-methylcytosine (Wyatt, 1951), 5-hydroxymethylcytosine (Wyatt & Cohen, 1953), and 6-methylaminopurine (Dunn & Smith, 1955, 1958a) in addition to adenine, guanine, cytosine and thymine. The first evidence of such additional bases naturally present in ribonucleic acid is the recent isolation of an unidentified nu...