Mena-lacZ (K18E2) expression. Overnight X-gal staining demonstrated high-level lacZ expression in undifferentiated ES cells (A) and in virtually all cells in the attached EB culture, including blood islands (bi) and their associated vasculature (arrow; B). Differentiation of clone K18E2 on OP9 stromal cells followed by overnight X-gal staining demonstrated high-level lacZexpression in mesodermal (M) colonies (C), whereas most hematopoietic cells did not express lacZ (short arrows), although low-level expression was observed in some isolated hematopoietic cells (long arrows; D). Mena-lacZ was expressed at high levels in vivo, as demonstrated by strong X-gal staining in less than 90 minutes in an e10.5 F₁ embryo (E). Overnight X-gal staining of an e13.5 F₁ embryo showed strong lacZ expression in all tissues except the liver (F).