The human HPRT gene on a yeast artificial chromosome is functional when transferred to mouse cells by cell fusion.

A 680-kb yeast artificial chromosome (YAC) that contains a functional copy of the human hypoxanthine phosphoribosyltransferase (HPRT) gene has been isolated. This YAC, yHPRT, and another YAC, yXY837, which contains the 3' end of the HPRT gene, have been mapped with restriction enzymes that cleave human DNA infrequently. The HPRT gene lies near the center of yHPRT. Fusion of yHPRT-containing yeast spheroplasts with mouse L A-9 cells, which are HPRT-negative, gives rise to HPRT-positive colonies. These colonies contain the human HPRT gene and express human HPRT mRNA. Fusion of yeast with mammalian cells is an efficient way of testing the integrity and functionality of human DNA contained in YACs.