Specifically, a patient who was compound heterozygous for the GOSR2 splice site variant c.336+1G>A and a novel c.364G>A, p.Glu122Lys missense variant presented with a global developmental delay and epilepsy at the age of 2, and clonazepam-responsive myoclonus at age 8—which is a radical departure from the typical sequence of symptom onset for North Sea PME. Here, GOSR2 is linked to Progressive myoclonic epilepsy.