To investigate the roles of BMP signaling at various stages of limb patterning and skeletogenesis, we constructed a series of mice deficient singly or in combination in the ability to produce BMP2, BMP4, and BMP7. BMP7-deficient mice (kindly provided by Dr. Liz Robertson) survive until birth. However, BMP2 and BMP4 are both required for viability early in embryonic development [30,31]. We therefore constructed a conditional allele of Bmp2, introducing loxP sites flanking exon 3. We obtained a conditional allele of Bmp4, in which exon 4 is flanked by loxP sites, from Dr. Holger Kulessa and Dr. Brigid Hogan [16]. Both of these alleles would be expected to result in null alleles following recombination (see Materials and Methods for details). To conditionally inactivate Bmp2 and Bmp4 in the limb, we used a well-characterized transgene in which cre-recombinase is expressed under the control of the Prx1 limb enhancer [32]. This transgene expresses cre very early in limb development, resulting in complete recombination of floxed alleles at early limb bud stages. We verified the ability of Prx1::cre to recombine the conditional Bmp2 and Bmp4 alleles at earliest limb bud stages using in situ hybridization. Bmp2 is first expressed in the limb mesenchyme at embryonic day (E)10.5 in the mouse (Figure 1A, asterisk). By the time its expression is first detectable, the floxed Bmp2 allele appears to be completely recombined in the limb mesenchyme, as whole mount in situ hybridization does not detect any mesenchymal Bmp2 transcription (Figure 1B). At E10.5 Bmp4 is expressed in the mouse limb mesenchyme in two stripes at the anterior and posterior margins (Figure 1C, red arrows). These expression domains are completely lost by E10.5 in the presence of the Prx1::cre transgene (Figure 1D). Bmp2 and Bmp4 are also expressed in the AER, where Prx1::cre is inactive, and these domains of expression are not affected (Figure 1A–1D, black arrows).