Fig. 1. Sequential expression of Mash1 and Ngn1 in progenitors of the olfactory epithelium. (A) In situ hybridisation with RNA probes for Mash1 (in blue) and Ngn1 (in brown) on frontal sections of OE at E12.5 shows that the two genes are expressed in distinct and partially overlapping progenitor populations. Mash1 is expressed in cells located in apical, intermediate and basal positions while Ngn1 expression is restricted to basal progenitors. The two genes are co-expressed in a subset of basal progenitors (arrowheads in the inset). The top right panel shows the apical (a) and basal (b) sides on a schematic representation of a OE section. (B,B') Hybridisation with a Ngn1 probe on E12.5 wild-type (B) and Mash1 mutant (B') OE showing that Ngn1 expression in basal progenitors requires Mash1 function. (C,C') Hybridisation with a Mash1 probe on E12.5 wild-type (C) and Ngn1 mutant (C') OE showing that in contrast, Mash1 expression in OE progenitors is independent of Ngn1 function. Scale bars, 25 µm.