The explant liver from our patient with OTC deficiency contains a hepatocellular adenoma. (a) The tumor is a well-circumscribed, soft, tan nodule (shown by arrows), measuring 2.0 cm. (b) At low power magnification, the nodule is homogeneous without thick fibrous bands (H&E, 40x). (c) The background liver shows glycogen changes with clear to pink cytoplasm and distinct cell membrane (H&E, 200x). (d) The nodule is composed of hepatocytes with normal N/C ratios (H&E, 100X). (e) Reticulin stain highlights the normal reticulin framework and shows the hepatocellular plates of overall 1–2 cells in thickness (200x). (f) Immunostaining for HepPar1 shows diffuse immunoreactivity (200x).