Cubilin, megalin and Folr1 protein expression in VYS. Expression of cubilin, megalin and Folr1 protein in the visceral yolk sac is shown in Figure 5. Cubilin (A), megalin (C) and Folr1 (E) protein are all highly expressed on the apical side of the visceral yolk sac in E9.5 RFC1+/+ embryos. Cubilin is also highly expressed on the apical side of the visceral yolk sac in E9.5 RFC1-/- embryos (B). Megalin expression, however, is completely absent from the visceral yolk sac of RFC1 mutants (D). Folr1 protein expression is absent from the apical plasma membrane of the VYS, but demonstrates increased expression in the endothelial cell layer of the yolk sac blood islands (F).