Sequences of 38 independent X-ray-induced mutations in a cDNA of the human hypoxanthine phosphoribosyltransferase (hprt) gene were analyzed after it was integrated into mammalian chromosomes as part of a shuttle vector. There were 15 base substitutions (39%), 9 frameshifts (24%), 11 deletions (29%), and 3 other mutations (8%) of two kinds. Radiation-induced mutations were found throughout the coding region of the gene. Base substitution mutations occurred more often at A:T sites than at G:C sites, and there were more transversions than transitions. Most of the frameshifts and deletions had short direct repeats or a run of several identical bases at the site of the mutation. A slippage misalignment mechanism, suggested previously, can account for the generation of these classes of mutations. The kinds, distribution, and possible mechanism of X-ray-induced mutations were similar to those of spontaneous mutations.