Arginase (ARG1) catalyzes the hydrolysis of arginine to form ornithine and urea. In chickens, the enzyme is localized in the mitochondrial matrix, is expressed at high levels in kidney, and only at low levels in liver (Kadowaki and Nesheim 1979; Tamir and Ratner 1963). Its physiological function may be to generate ornithine as a first step in the synthesis of polyamines (Grillo et al. 1983); the fate of the urea that is also generated is unknown. The ARG1 gene has not been directly studied but rather has been identified computationally in the ENSEMBL chicken gene set as a single open reading frame capable of encoding a protein similar to human ARG1.