FAS and autoimmune lymphoproliferative syndrome: <h4>Background</h4>The autoimmune lymphoproliferative syndrome (ALPS) is a rare disease characterized by defective function of the FAS death receptor, which results in chronic, non-malignant lymphoproliferation and autoimmunity accompanied by elevated numbers of double-negative (DN) T cells (T-cell receptor α/β + CD4-CD8-) and an increased risk of developing malignancies later in life.<h4>Case description</h4>Here, we report a patient with a <i>de novo</i> FAS mutation with a severe phenotype of ALPS-FAS.