To examine whether the successful execution of chondrogenesis in the BMP2, BMP4–deficient limbs is a result of compensatory upregulation of other ostegenic BMPs, we carried out in situ hybridization for Bmp6 and Bmp7 in these limbs. At E13.5, we observed no increase in expression of Bmp6 (Figure S1P–S1S) or Bmp7 (compare Figure 4H with Figure 4G) mRNA in the mutant tissues compared to their wild-type counterparts