PEPD and Autoimmunity: Biallelic homozygous or compound heterozygous loss-of-function mutations in PEPD, the gene encoding prolidase, cause an autosomal recessive inborn error of immunity (IEI), prolidase deficiency (PD), falling under diseases of immune dysregulation and in particular, under the subgroup of autoimmunity.3 4 Typical manifestations of PD include dysmorphic features, chronic skin ulcers, recurrent infections and features of autoimmune connective tissue diseases.