Figure 1.

(A) Schematic illustration of the major approaches for retrospective clonal analysis in the mouse. These include: making aggregated embryos between genetically distinct strains at the 4- to 8-cell stage (12, 13); injecting genetically tagged ES cells into blastocyst embryos (14); taking advantage of random X chromosome inactivation of transgene in female mice (15); and infecting embryos with replication-defected retrovirus (16–18). The timing of labeling clones in each method also is indicated in perspective. In the present study, D3 ES cells were first transfected and selected for stable expression of the lacZ gene and then used for injection into mouse blastocyst embryos. As shown in B, colonies of undifferentiated ES cells (actin 69) display homogeneous expression of the lacZ gene on top of a fibroblast feeder cell layer in vitro.