The study of limb development has provided insight into pattern formation during vertebrate embryogenesis. Genetic approaches offer powerful ways to identify the critical molecules and their pathways of action required to execute a complex morphogenetic program. We have applied genetic analysis to the process of limb development by studying two mouse mutants, limb deformity (ld) and Strong's luxoid (lst). These mutations confer contrasting phenotypic alterations to the anteroposterior limb pattern. The six mutant ld alleles are fully recessive and result in oligosyndactyly of all four limbs. By contrast, the two mutant lst alleles result in a mirror-image polydactylous limb phenotype inherited in a semidominant fashion. Morphological and molecular analysis of embryonic limbs has shown that the ld and lst alleles affect the extent and distribution of two key signaling centers differentially: the apical ectodermal ridge and the zone of polarizing activity. Molecular characterization of the ld gene has defined a new family of evolutionarily conserved proteins termed the formins. The underlying molecular defect in the lst mutation has not been identified; however, both loci are tightly linked on mouse chromosome 2, suggesting the possibility that they may be allelic. In this study, we have used genetic analysis to examine the epistatic and allelic relationships of ld and lst. We observed that in + ld/lst + double heterozygotes, a single mutant ld allele is able to suppress the semi-dominant polydactylous lst limb phenotype. By segregating the lst and ld loci in a backcross, we observed that these loci recombine and are separated by a genetic distance of approximately 6 cM. Therefore, while our observations demonstrate a genetic interaction between ld and lst, it is probable that ld and lst are not allelic. Instead, lst and ld may be operating either in a linear or in a parallel (bypass) genetic pathway to affect the limb signaling centers.