Parkinson's disease (PD) is a common neurodegenerative disorder in the aging 
population, affecting more than 1% over the age of 65 years. Certain rare forms 
of the disease are monogenic, representing 5-10% of PD patients, but there is 
increasing evidence that multiple genetic risk factors are important also for 
common forms of PD. To date, 13 genetic loci, PARK1-13, have been suggested for 
rare forms of PD such as autosomal dominant and autosomal recessive PD. At six 
of these loci, genes have been identified and reported by several groups to 
carry mutations that are linked to affected family members. Genes in which 
mutations have been linked to familial PD have also been shown to be candidate 
genes for idiopathic forms of PD, as those same genes may also carry other 
mutations that merely increase the risk. Four of the PARK genes, SNCA at PARK1, 
UCH-L1 at PARK5, PINK1 at PARK6 and LRRK2 at PARK8, have been implicated in 
sporadic PD. There are indeed multiple genetic risk factors that combine in 
different ways to increase or decrease risk, and several of these need to be 
identified in order to begin unwinding the causative pathways leading to the 
different forms of PD. In this review, we present the molecular genetics of PD 
that are understood today, to help explain the pathways leading to 
neurodegeneration.