To label embryonic stem (ES) cells with enhanced green fluorescent protein (EGF P) on the hypoxanthineguanine phosphoribosyl transferase (HPRT) gene locus for t he first time to provide a convenient and efficient way for cell tracking and ma nipulation in the studies of transplantation and stem cell therapy Methods Homologous fragments were obtained by polymerase chain reaction (PCR), from whic h the gene targeting vector pHPRT EGFP was constructed The linearized vector was introduced into ES cells by electroporation The G418 r6TG r cell clones were obtained after selection with G418 and 6TG media The integration patterns of these resistant cell clones were identified with Southern blotting Results EGFP expressing ES cells on the locus of HPRT were successfu lly generated They have normal properties, such as karyotype, viability and di fferentiation ability The green fluorescence of EGFP expressing cells was main tained in propagation of the ES cells for more than 30 passages and in different iated cells Cultured in suspension, the “green” ES cells aggregated and forme d embryoid bodies, retaining the green fluorescence at varying developmental sta ges The “green” embryoid bodies could expand and differentiate into various t ypes of cells, exhibiting ubiquitous green fluorescence Conclusions This generation of “green” targeted ES cells is described in an efficient proto col for obtaining the homologous fragments by PCR Introducing the marker gene in the genome of ES cells, we should be able to manipulate them in vitro and use them as vehicles in cell replacement therapy as well as for other biomedical a nd research purposes To label embryonic stem (ES) cells with enhanced green fluorescent protein (EGF P) on the hypoxanthineguanine phosphoribosyl transferase (HPRT) gene locus for he he first time to provide a convenient and efficient way for cell tracking and ma nipulation in the studies of transplantation and stem cell therapy Methods Homologous fragments were obtained by polymerase chain reaction (PCR), from whic h the gene targeting vector pHPRT EGFP was constructed The linearized vector was introduced into ES cells by electroporation The G418 r6TG r cell clones were obtained after selection with G418 and 6TG media The integration patterns of these resistant cells clones were identified with Southern blotting results EGFP expressing ES cells on the locus of HPRT were successfu lly generated They have normal properties, such as karyotype, viability and di-differentiation ability The green fluorescence of EGFP expressing cells was main tained in propagation of the ES cells for more than 30 passages and in different iated cells Cultured in suspension, the “green ” ES cells aggregated and forme d embryoid bodies, retaining the green fluorescence at varying developmental sta ges The “green ” embryoid bodies could expand and differentiate into various t ypes of cells, exhibiting ubiquitous green fluorescence Conclusions This generation of “green” targeted ES cells is described in an efficient proto col for obtaining the homologous fragments by PCR Introducing the marker gene in the genome of ES cells, we should be able to manipulate them in vitro and use them as vehicles in cell replacement therapy as well as for other biomedical a nd research purposes