<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by guanidinoacetate methyltransferase deficiency ? [/INST] Guanidinoacetate methyltransferase deficiency is a very rare disorder. About 80 affected individuals have been described in the medical literature. Of these, approximately one-third are of Portuguese origin. </s>