Cleft lip and palate (CLP) is a common human birth defect that develops when the tissues of the lip and palate don't fuse properly during embryonic development. It affects 1 in 700 babies and may result from environmental and/or genetic factors. This research is focused on the identification of mutations in candidate genes in the affected twin of a discordant monozygotic (MZ) pair. Detecting these mutations can provide information about the causes of the phenotypic differences between the pairs. There are a number of reasons that cause discordance in MZ twins: postzygotic mutations, environmental differences and X chromosome inactivation. We did zygosity testing in 14 pairs of twins by genotyping 20 SNPs. We found that 8 pairs were MZ while 6 of them were dizygotic (DZ). Two of the MZ pairs were discordant for CLP. The 2 MZ discordant pairs identified in this study plus 14 other discordant MZ pairs were selected for sequencing of candidate genes BMP4 and IRF6. DNA samples were amplified using the PCR technique and then cycle sequenced. We successfully determined the zygosity of 14 MZ twins. Sequencing results are pending. Further studies will determine if there is a mutation that causes CLP.