Post-implantation development requires precisely coordinated cellular movements in preparation for gastrulation. We have characterized and genetically mapped a spontaneous X-linked mouse mutant, Polypodia (Ppd), which we discovered and established as a mutant line using in vitro fertilization and genetic crosses to explore early post-implantation body plan patterning. Ppd mice exhibit ectopic caudal limbs and other extraordinary malformations, which has been observed in humans and other animals. We hypothesize that Ppd alters vertebrate patterning during pregastrulation or early gastrulation stages by changing expression of patterning genes in the post-implantation embryo. Because of the strikingly similar malformations of Ppd mice by comparison with embryos treated with exogenous retinoic acid at E4.5-E5.5, we hypothesize that Ppd malformations are secondary to alterations of primitive streak formation or mesodermal cell allocation via premature activation of retinoic acid synthesis or alterations of Wnt/β-catenin signaling. We are determining the timing and nature of lethality and testing for alterations in post-implantation gene expression using RNA probes for genes expressed in the primitive streak and proximal epiblast and overlying visceral endoderm; distal visceral endoderm; extraembryonic ectoderm; and epiblast. To determine whether induced alterations in gene expression occur in common between Ppd and RA-treated mice, we are comparing RA-treated pregnant wild-type mouse embryos at E4.5-E5.5 to Ppd mutant mice. We are studying the functional consequences of the mutation using transgenic mice and other tools.