Isl1 is expressed in a subset of early Mesp1-expressing cells. (A and B) Quantification of Mesp1-GFP (A) and Isl1 (B) expression as measured by immunostaining of GFP and Isl1 on cytospin slides of Mesp1-GFP cells at D3 and D4. n = 3. (C and D) Confocal microscopy analysis of GFP (Mesp1) and Isl1 immunostaining in Mesp1-GFP cells at D3 (C) and D4 (D). (right) Magnification of the insets, and arrows indicate cells that coexpress Mesp1 and Isl1. Bars, 30 µm. (E and F) Quantification of Isl1 expression in Mesp1-GFP–expressing cells (E), and Mesp1 (GFP) expression in Isl1-expressing cells (F) at D3 and D4. More than 300 cells were counted in each condition. n = 3. Error bars indicate means ± SEM.