The his4-912 mutation results from insertion of a 6200 bp transposable element into the his4 gene of yeast. In order to clone the his4-912 mutation, the plasmid pBR322 was integrated into the his4 gene by means of yeast transformation, and then the vector sequences and the his4-912 insertion element were excised as a single restriction fragment. This his 4-912 insertion element is homologous to Ty1, a family of repetitive yeast DNA sequences. His+ revertants derived from the his4-912 mutant carry a number of chromosomal aberrations including deletions, translocations, a transposition and an inversion. The majority of His+ revertants result from deletions which have both endpoints within the element and which leave behind only 300 bp of the insertion element. Other derivatives of the his4-912 mutant carry deletions which have one endpoint in the insertion element and one endpoint in the his4 coding sequence. In two His+ revertants carrying reciprocal translocations, the chromosome III translocation breakpoints occur within the his4-912 insertion element. A His+ revertant carrying an inversion of most of the left arm of chromosome III may be an intermediate in transposition of the his4-912 insertion element to a new site on chromosome III.