Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by vitamin D-dependent rickets ?
    ### Response:
    Rickets affects an estimated 1 in 200,000 children. The condition is most often caused by a lack of vitamin D in the diet or insufficient sun exposure rather than genetic mutations; genetic forms of rickets, including VDDR1 and VDDR2, are much less common. The prevalence of VDDR1 and VDDR2 is unknown.
    