PNP and hyperinsulinemic hypoglycemia, familial, 4: Such studies have already been recently published for Alzheimer disease [52], purine nucleoside phosphorylase deficiency [53], Friedreich ataxia [44] and very recently cyclin-dependent kinase-like 5 (CDKL5) Deficiency Disorder [18], a disorder that shares similar symptomatic features with RTT.