Fig. 6. The BL-CFC defect in Arnt^–/– cells is cell extrinsic. (A) Arnt^+/+ neo^s lacZ^– and Arnt^–/– neo^r lacZ⁺ ES cells co-cultured during EB differentiation, dissociated and replated in methylcellulose with or without G418. (B) Under G418 selection, all surviving BL-CFCs stained blue indicating ß-galactosidase activity from Arnt^–/– neo^r lacZ⁺ cells. Wild-type clones only survived in the absence of G418.