Although monogenic forms represent only a small subset of PD cases, several genetic factors—including nucleotide repeat expansions (NREs) in ATXN2, ATXN3, C9ORF72, TBP, POLG, TOMM40, CACNA1A, and PRNP—have been implicated in neurodegenerative conditions with parkinsonian features. Here, C9orf72 is linked to Parkinson disease.