A hepatic porphyria caused by monoallelic and biallelic variants in HMBS and presenting as a spectrum of disease (a semidominant inheritance pattern). Monoallelic variants predispose to acute/episodic attacks in adulthood with abdominal pain, neuropathy, and neuropsychiatric symptoms (women are more often affected) without cutaneous manifestations. Triggers precipitating acute attacks include estrogen/progesterone, oral contraceptives, alcohol, drugs, stress, or infections. Biallelic variants cause severe disease in childhood presenting with neurological issues including developmental abnormalities, ataxia, dysarthria, leukoencephalopathy, cataracts and optic nerve hypoplasia.