While certain gene mutations (e.g. PARKIN mutations) are associated with a more favourable disease course in PD, other gene mutations including glucocerebrosidase (GBA) mutations52–54 and the presence of the MAPT H1/H1 genotype have been found to be associated a more rapid course of cognitive decline in PD patients and a higher risk of developing PDD.55,56 As genetic testing was not undertaken in this study, it is impossible to know the possible impact of such effects. Here, GBA1 is linked to Parkinson disease.