As noted above, at E12.5, chondrogenic condensation of the digits could be visualized in Bmp2C/C; Bmp4C/C; Prx1::cre limb buds by in situ hybridization with a Sox9 probe. Similar analysis at E10.5 and E11.5 demonstrated a relatively normal process of condensation of the proximal skeletal elements (Figure S1H–S1M), despite the absence of detectable Bmp2 and Bmp4 transcripts. As shown in Figure S1N and S1O, expression of Col II, a chondrocyte marker, is observed in the BMP2, BMP4–deficient cartilage (Figure S1O) as early as E12.5, similar to a wild-type control (Figure S1N). Similarly, at E13.5, the entire limb skeleton could be visualized undergoing chondrogenesis by Alcian blue staining, albeit in a defective pattern, in the BMP2, BMP4–deficient limb buds (Figure 4A and 4B and unpublished data).