APOE and REM sleep behavior disorder: Indeed, recent findings support the possible usefulness of hyposmia as a prodromal biomarker because it is present in some clinically normal GBA mutation carriers [45], is common in idiopathic REM sleep behavior disorder [46,47] and in older adults carrying the APOE ε4 allele, which is the most robust genetic risk allele for AD [49], DLB [50] and DLB spectrum [51] independently of AD pathology [52].