Provides the first demonstration of germ-line transmission and expression of a large human DNA fragment, introduced into embryonic stem (ES) cells by fusion with yeast spheroplasts. Yeast artificial chromosomes (YAC); Human hypoxanthine phosphoribosyltransferase (HPRT) gene; Characterization of yHPRT and yeast genomic DNA integrated in ESY clones; Determination of the presence of yeast genomic DNA in the ESY clones; More.