Given recent reports that cimetidine improved cutaneous photosensitivity in patients with EPP, presumably by inhibiting ALAS activity in erythroid cells [36,37,38], we investigated whether cimetidine inhibits ALAS2 activity, first in vitro and then in bone marrow cells isolated from cimetidine-treated mice. The gene discussed is ALAS1; the disease is autosomal erythropoietic protoporphyria.