Dear Sir, Tyrosinase (Tyr) is the key enzyme in melanin synthesis, involved in the conversion of tyrosine to dopaquinone. It was long thought to be specific to pigmented cells. This is certainly true in adults. However, during mouse development, the Tyr promoter is active in melanoblasts and cells along the entire neural tube (Tief et al., 1996). Transgenic mice producing the Cre recombinase under the control of the tyrosinase promoter (Tyr:: Cre) were previously generated to allow Cre-mediated recombination of genes in melanocytes (Mc)(Delmas et al., 2003). The Cre protein is produced in classical and non-classical Mc during late embryogenesis in these mice (Yajima and Larue, 2008). Crossing Tyr:: Cre mice with Rosa26flox reporter mice revealed recombination events in other non-pigmented tissues (Delmas et al., 2003; Tonks et al., 2003; Yajima et al., 2006). The finding of Tyr activity in cells along the neural tube indicated that Cre recombinase in Tyr:: Cre mice is expressed in cells other than melanoblasts. Vagal neural crest cells (VNCC) can differentiate into various cells and tissues such as Mc, smooth muscle cells (Ms), neurons (N), glial cells and mesenchymal cells (Le Douarin and Kalcheim, 1999). At least two major events occurring after birth are associated with VNCC derivatives: modification of the blood stream after birth, requiring the closure of the ductus arteriosus (DA) and modification of the food supply at weaning, requiring the proper transit of solid feces including fiber into the intestine.

The Tyr promoter described previously (Porter and Meyer, 1994) is active from E10. 5 in NCC of the truncal region (Delmas et al., 2003). The examination of transverse sections obtained from Tyr:: Cre⁄; Rosa26flox⁄+ and Dct:: LacZ E10. 5 embryos revealed that not only melanoblasts were defloxed (Figure S1), but also that these defloxed cells could be at the origin of a subset of DA and intestinal cells.