PRKN and Parkinson disease: LRRK2 Gly2019Ser-associated PD can be easily distinguished from PARK2-associated PD on the basis of an older age of onset; the symptoms of PD were rare before 40 years in patients with LRRK2 Gly2019Ser-associated PD, whereas symptom onset after 40 years was rare in patients who were homozygous for mutations in PARK2. This rule of thumb has clinical uses, particularly in the genetic counselling of patients with sporadic PD and their affected siblings, in whom the mode of inheritance is often unclear.