These observations have direct biological and clinical relevance. First, they support previous efforts to conceptualize Tourette and chronic tics as a unified condition and to combine them into a single tic spectrum disorder in future diagnostic schemas (1). Although traditionally separated clinically into distinct disorders, chronic/persistent tic disorders, whether consisting of motor tics, vocal tics, or both, appear to be due to the same underlying genetic causes. Second, while the small proportion of explained variance in worst-ever tic severity is a limitation of the current study, work in other polygenic psychiatric disorders such as schizophrenia has repeatedly demonstrated that, as GWAS sample sizes increase, the proportion of phenotype explained by polygenic risk scores increases markedly (30). It is therefore possible that in the future, Tourette PRS might be a potential candidate for predicting both conversion to chronic tics in the 20-25% of children who present with transient tics (1), and, at the other end of the phenotypic spectrum, tic persistence and lifetime tic severity in those with Tourette syndrome. Finally, particularly important in the context of the very large sample
