GBA1 and Parkinson disease: Even though there is evidence supporting loss‐ and gain‐of‐function of GCase, both hypotheses have limitations, as presence of null mutations in some PD‐GBA1 patients contradicts the gain‐of‐function theory, while development of PD‐GBA1 in individuals carrying heterozygous GBA1 mutations contradicts the loss‐of‐function theory.