Gene trapping is a useful method to identify new genes involved in development. Here we describe the spatiotemporal expression of a gene identified in a gene-trap screen. This gene is first expressed at 9.5 days postcoitum (E9.5) in the forelimbs and in the branchial arches region. At E11.5, expression was detected in the stomach, genital bud, and pharyngeal epithelium. At later stages, expression includes the hair follicles, whereas the expression in the stomach and pharynx disappears. We performed 5'-rapid amplification of cDNA ends (RACE) to amplify and clone a partial cDNA of the endogenous sequence fused to the lacZ reporter gene. The sequence did not reveal any similarity to known sequences and was named paddy. The expression pattern suggests multiple roles during limb development. The early phase of expression, for instance, correlates with anteroposterior (A/P) regionalization. In contrast to other molecules involved in A/P polarization, paddy expression fades away distally as the bud elongates. This suggests that expression of paddy in late stages does not depend on apical ectodermal ridge (AER) and zone of polarizing activity (ZPA) signaling and is probably involved in posterior determination in more proximal regions of the limb.