SOX9 and Bartsocas-Papas syndrome 1: The absence of Sox9 expression in mouse BPs (Figure 1), which are known to have a low proliferative capacity, vs. its presence in the highly proliferative human BPs (Figure 2) and its requirement for the abundance of the highly proliferative ferret BPs (Figure 3), led us to hypothesize that Sox9 could promote BP proliferation.