In everyday medical practice, parameters such as Crohn’s disease activity indices (CDAI) for patients with CD, the Truelove–Witts score for patients with UC, cell counts (white blood cells and absolute neutrophil count), and serum transaminases (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)) are analyzed to estimate thiopurine treatment efficiency and adverse effects. This evidence concerns the gene GPT and Crohn disease.