disease development. An example of the clear phenotypic variability is a monozygotic twin study conducted by Jobling et al. (2010). Two monozygotic female twins had the same IRF6 mutation; however Twin A was born with a bilateral cleft lip, whereas Twin B had bilateral lip pits and no orofacial clefting. Both twins were diagnosed with VWS. The twins' father had lip pits alone and a family history of CLP, CP, CL, and/or lip pits. Both twins were diagnosed with VWS. Polymerase chain reaction (PCR) amplification was done for all exons of IRF6, and a missense mutation was discovered in exon 4. The tyrosine in the normal protein at this position is conserved across mammals, frogs, and chickens, so despite the fact that it was a previously unreported mutation, it was expected to be deleterious. This study is not the first case of different phenotypes occurring between monozygotic twins. Possible causes of phenotypic variability include variations in the intrauterine environment, epigenetic differences, or chance effects. Pathophysiology Lip pits were first reported in 1845 by Demarquay who thought they were indentations made by the upper incisors. Today it is known that they occur during embryogenesis due to IRF6 mutations. On day 32 of embryonic development there are four growth centers of the lower lip divided by two lateral grooves and one median. By day 38 of normal development, the grooves have disappeared. However, when growth of the mandibular process is impeded, a lip pit occurs. Lip pits begin to develop on day 36. Cleft lip begins to develop on day 40, and cleft palate emerges on day 50 of development. There are three types of lip pits, which are classified according to their location: midline upper, comissural, and lower lip. The most common phenotype is two symmetrical lower lip pits flanking both sides of the midline in the bilateral paramedial sinuses. Lower lip pits may also be bilaterally, unilaterally, or medially asymmetrical. The occurrence of a single lip pit is considered incomplete expression, and it typically occurs on the left side of the lower lip. There are also three different shapes for lip pits, the most common being circular or oval; less common forms include slit-like or transverse. The lip pits extend into the orbicularis oris muscle, ending in blind sacs surrounded by mucous glands. In some cases mucous is excreted when the muscles contract. Diagnosis Clinical diagnosis based on orofacial clefts