Multiple copies of a transgenic construct, consisting of the Roux Sarcoma Virus promoter adjoined to the entire coding region of the human placental alkaline phosphatase gene, inserted in tandem array in the middle of chromosome 5. Phenotypic characteristics segregated with transgene and were not observed in a different line with similar levels of expression, indicating the phenotype was largely due to the site of insertion. A non-complementation test demonstrated that the insertional mutation is allelic to pirouette. Further molecular analysis showed that the insertion is located in intron 1, 32 kb upstream of exon 2.