Expression of the human EpoR in the mouse EpoR locus.

Primer extension of reverse transcriptase–polymerase chain reaction (RT-PCR) product was used to evaluate relative levels of mouse and human EpoR mRNAs, as described.2 The genotypes of ES cells (first lane) or mice (all other lanes) are indicated. The ES cells in the first lane were in vitro–differentiated into embryoid bodies in semisolid media with Epo.7 The cultures were harvested at day 9 of differentiation, and the cells were used as a template for RT-PCR. The presence of a neo-selectable marker did not suppress human EpoR gene expression in the mouse EpoR locus in vitro. In the other lanes, bone marrow cells were used as a template for RT-PCR. The retention of a neo cassette in the human EpoR gene generated a null or hypomorphic allele in vivo. The presence (+neo) or absence (Δ neo) of this selectable marker gene is displayed. wt indicates wild type; mt, mutant.