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Another potentially useful approach to TS genetic research was illustrated by \citet{27708560}, who sequenced candidate genes suggested by previous research in order to identify potential functional variants linked to TS.  ### Epigenetics  A review of epigenetic mechanisms and how they may prove to affect TS appeared in mid-year \cite{27462201}.  ### Environmental risk factors and epidemiology  Identifying specific environmental risk factors for TS is important in its own right and also because controlling for known environmental risks can improve the power of genetic studies. Previous studies have linked TS with lower birthweight, and more recently with maternal smoking during gestation \cite{23219489,24262815,24479407}. Leivonen et al. reported the first nationwide pseudo-prospective (_i.e._, based on a Finnish national health registry) study of prenatal maternal smoking and TS \cite{25796373}. In this sample the hypothesized association of maternal smoking with TS _per se_ was not significant, but maternal smoking during pregnancy was significantly associated with combined TS and ADHD, and the odds ratio for TS+ADHD following first trimester smoking was 4.0 (95% CI 1.2-13.5, p=.027). On the one hand, this association provides only modest support for the link between prenatal toxicity and TS. However, this study is consistent with the idea shared by many tic experts that the current nosology artificially separates tics from other common symptoms in people with TS.