LRRK2 and Parkinson disease: In contrast, our analysis showed that leucine and tyrosine were also decreased in LRRK2 PD plasma samples, while previous studies suggested that leucine is increased in the CSF, fecal and urine samples of patients with PD [55,56,57] and increased tyrosine levels were found in the CSF, urine, and saliva of PD patients [55,58,59].