Fah–/– mice, a model of HT1, universally develop hepatitis, failure to thrive, and death in the absence of nitisinone (NTBC), a repurposed herbicide that blocks the activity of 4-hydroxyphenylpyruvate dioxygenase (HPD) in the tyrosine catabolic pathway (Figure 4A). The gene discussed is HPD; the disease is hepatitis A virus infection.