HAAO and metabolic dysfunction-associated steatohepatitis: Additionally, publicly available RNA‐seq data revealed that the mRNA levels of tryptophan 2,3‐dioxygenase (TDO2), 3‐hydroxyanthranilic acid dioxygenase (HAAO), and quinolinate phosphoribosyl transferase (QPRT), encoding key enzymes in the de novo and salvage biosynthesis pathway for NAD+ synthesis, were decreased in the liver samples from patients with non‐alcoholic steatohepatitis (GSE49541)[17] or alcoholic hepatitis (GSE28619)[18] (Figure S2C–E, Supporting Information).