PRKDC and severe combined immunodeficiency: Other disorders involving the NHEJ system also can be considered as differential diagnoses, since pathogenic variations impacting PRKDC [MIM*600899], NHEJ1 [MIM*611290], and LIG4 [MIM*601837] can manifest as MPD with neurodevelopmental disorder, but in these syndromes, severe combined immunodeficiency (SCID) is at the forefront, in contrast to XRCC4-related MPD.