Apart from PPM1B, three other genes are affected: the full type I cystinuria gene SLC3A1, the full, mostly uncharacterised, serine peptidase-encoding gene PREPL, and the first exon of the class I protein methyltransferase gene C2orf34. Patients present with cystinuria, neonatal seizures, hypotonia, severe somatic and developmental delay, facial dysmorphisms, and lactic acidemia. Here, PPM1B is linked to cystinuria.