Tbx genes have important roles in mammalian embryogenesis. In particular, mutations in TBX3 and TBX5 cause Ulnar-mammary and Holt-Oram syndromes in humans, respectively. These transcription factors are expressed in the early stages of limb development and disruption of these genes in the aforementioned syndromes results in variable limb defects. Importantly, Tbx3 is the only known Tbx gene expressed in both the early limb mesenchyme and in the Apical Ectodermal Ridge (AER), which is a crucial signaling center directing limb outgrowth. It has previously been shown that Tbx3 null mouse mutants die in mid gestation, which prevents a complete analysis of the roles of this protein in limb development. Therefore, we developed a conditional mutagenesis system using gene targeting in mice. This system employs conditional alleles of Tbx3 and a series of Cre-recombinase expressing drivers which permits us to perform tissue- specific ablation of Tbx3 function in the AER versus the limb mesenchyme, and as compared with ablation in all limb precursors. This approach allows us to interrogate the expression domain-specific roles of Tbx3 in gene regulation during limb morphogenesis. We will present anatomic and molecular phenotypes of these domain-specific mutants and offer hypotheses about the potential regulatory roles played by Tbx3 during normal and abnormal limb morphogenesis.