Immunodeficiency with hyper IgM type 2 is caused by a mutation in the Activation-Induced Cytidine Deaminase (AICDA) gene, which is located on the short arm of chromosome 12.
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Immunodeficiency with hyper IgM type 5 is caused by a mutation in the Uracil-DNA glycosylase (UNG) gene, which, like AICDA, is located on chromosome 12.