Sir,Trisomy 13 syndrome occurs in approximately 1/5000 live births. It is most commonly caused by nondisjunction. Robertsonian translocation involving chromosome 13 with another acrocentric chromosome in an unbalanced state accounts for 5%-10% of trisomy 13 cases. In this situation there is an increased risk of familial recurrence, however the phenotype is the same. We report a case of monozygotic female twins. Both inherited as an unbalanced 13;15 translocation from their father. The trisomy 13 constitution was present in 100% of the cells analyzed on both girls. The phenotype however was more severe in twin B than twin A. Twin B had proboscis and cyclopia. Twin A had microphthalmia and hypotelorism. The cause of this varying phenotype is unknown. Possible causes include environmental factors, chromosome X-inactivation, post zygotic gene mutations, circulatory changes due to placental vascular anastomoses or exogenous factors such as infections, drugs, nutrition or oxygen therapy.[12]