Infantile-onset inflammatory bowel disease (IO IBD) is an invalidating illness with an onset before 2 years of age and has a complex pathophysiology in which genetic factors are important. Here using homozygosity mapping and whole-exome sequencing, Desiree Haaften-Visser identifies for the first time mutations in the ANKZF1 gene in IO IBD patients. Although the function of ANKZF1 in mammals had not been previously evaluated, ANKZF1 was found to have an indispensable role in the mitochondrial response to cellular stress. ANKZF1 is located diffusely in the cytoplasm and translocates to the mitochondria upon cellular stress. ANKZF1 depletion reduces mitochondrial integrity and mitochondrial respiration under conditions of cellular stress. The ANKZF1 mutations identified in IO IBD patients results in dysfunctional ANKZF1, as shown by an increased level of apoptosis in patients' lymphocytes, a decrease in mitochondrial respiration in patient fibroblasts with a homozygous ANKZF1 R585Q mutation, and an inability of ANKZF1 R585Q and E152K to rescue the phenotype of yeast deficient in Vms1, the yeast homologue of ANKZF1. These data indicate that loss-of-function mutations in ANKZF1 result in deregulation of mitochondrial integrity, and this may play a pathogenic role in the development of IO IBD. This work has been recently published in the Journal of Biological Chemistry.