Fig. 4. Expression of heart markers. (A,B) Nkx2.5 expression at E7.75. At this stage Nkx2.5 is normally expressed in the heart precursor cells that form a crescent surrounding the heart field (A). The crescent shaped region of Nkx2.5 expression is absent in a representative mutant embryo shown in B. (C,D), Mesp1 expression in gastrulating embryos. In the wild-type E6.75 embryo, Mesp1 expression is seen in early ingressing mesodermal cells along the length of the primitive streak (C). An abnormal V-shaped expression pattern of the Mesp1 gene in early ingressing mesodermal cells is observed in a stage-matched Ldb1 null mutant embryo (D; posterior is facing up). The arrows in B and D indicate the embryonic-extraembryonic junction.