Frameshift indel mutations in exon 11 of the human ALAS2 gene (on chromosome Xp11.21) that result in deletion, replacement, or elongation of its Ct-extension are the molecular cause of X-linked protoporphyria (XLP, MIM 300752), an inherited disorder that presents with painful phototoxicity and an increased risk for liver dysfunction and failure21,22, due to high levels of the toxic heme intermediate PPIX. Here, ALAS2 is linked to X-linked lymphoproliferative disease.