A 7-year-old girl was admitted to the hospital for anaemia, secondary to 
intestinal blood los (melaena). She was found to have 45,X Turner's syndrome. 
Her identical twin sister also had Turner's syndrome with a 45,X chromosome 
complement. According to various criteria the probability of monozygosity was 
0.9905. Although the incidence of twinning is greater than usual in families of 
patients with Turner's syndrome, affected cases have only been observed in twin 
sisters on six occasions. It seems therefore that the 45,X chromosome complement 
itself is not a factor predisposing to twinning, but that in some families, a 
factor is at play, which cuases either twinning or the 45,X aneuploidy, or both.