<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by nonsyndromic holoprosencephaly ? [/INST] Nonsyndromic holoprosencephaly accounts for approximately 25 to 50 percent of all cases of holoprosencephaly, which affects an estimated 1 in 10,000 newborns. </s>