Insertion mutations in yeast caused by the transposable element Ty have been identified at several genetic loci. In all cases so far, the site of Ty insertion has been in the 5' non-coding region of the affected gene. Experiments presented here demonstrate that the ura3-52 mutation, a non-reverting ura3 mutation, is caused by a Ty insertion mutation within the coding region of the URA3 gene. This Ty insertion within a gene has a similar structure to those in non-coding regions.