Although the level of serine 1292-phosphorylated LRRK2 [Ser(P)-1292 LRRK2] has been shown to be elevated in the urine exosomes of PD patients, several unanswered questions remain that make it difficult to determine whether Ser(P)-1292 LRRK2 is a useful biomarker. This evidence concerns the gene LRRK2 and Parkinson disease.