The mutational specificity of the base analog 6-N-hydroxylaminopurine (HAP) was studied in the URA3 gene of the yeast Saccharomyces cerevisiae. Twenty-nine independent HAP-induced ura3 mutations were sequenced. GC-->AT transitions were found most frequently (21 out of 29) while AT-->GC transitions were less abundant (five out of 29). Three GC-->TA transversions were also detected. Two interesting features of DNA context were revealed for transition mutations. One third of the transitions occurred at one site within short direct imperfect repeats converting them to perfect repeats. A model involving complementary interaction of imperfect repeats is proposed to explain the origin of these mutations. Nearly all of the rest of the GC-->AT as well as the AT-->GC transitions were found in the runs of several identical base pairs, predominantly in the middle or at the 3' template nucleotide of (G)n and (A)n runs.