Previous unsuccessful attempts to induce CH fate with BMP4 (Furuta et al., 1997; Hu et al., 2008; Monuki et al., 2001) may relate to the early and short competency period for CH fate in vivo (E8.5-E10.5 in mice; Mangale et al., 2008). This evidence concerns the gene C4B and cyclic hematopoiesis.