GDNF and tic disorder: While there is no study of NGF and GDNF in tic disorders, in a study investigated catecholaminergic genes there was a significant correlation between the BDNF gene expression and the severity of tic.18 In another study, there was no relationship between BDNF polymorphisms and tic severity.19 Similar to this study, no correlation was found between NGF and GDNF and tic severity, in our study.