Although absent in undifferentiated hESCs, eGFP fluorescence directed from the T/brachyury promoter vector (Figure 1A) was detectable in differentiating mesenchymal outgrowths surrounding hESC colonies (Figures 5A–D) permitting enrichment and isolation of positive colonies after infection by the clonal isolation method described in Figure 4, or by using a drug resistance procedure described in the following section. The gene discussed is TBX1; the disease is infection.