<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by hereditary hypophosphatemic rickets ? [/INST] X-linked hypophosphatemic rickets is the most common form of rickets that runs in families. It affects about 1 in 20,000 newborns. Each of the other forms of hereditary hypophosphatemic rickets has been identified in only a few families. </s>