At the time of this review, one of the only studies investigating maternal cigarette smoking during pregnancy and miRNA expression was conducted by Maccani and colleagues and used a candidate miRNA approach to investigate the changes in four human placental miRNA associated with maternal cigarette smoking during pregnancy (Maccani et al., 2010). The four miRNA were selected for investigation because they had been previously reported to be expressed in the placenta and had been shown to be involved in the regulation of key cell processes. The work identified the association of maternal cigarette smoking during pregnancy with the downregulation of three specific miRNA: miR-16, miR-21, and miR-146a. Based on these findings, this research was expanded to consider three placental cell lines from different stages of placental development. The aim was to further investigate the effects of nicotine and benzo(a)pyrene, two components of cigarette smoke previously suggested to have negative effects on both the placenta and fetus, on miRNA expression in placental cells. Downregulation of miR-146a in one placental cell line, TCL-1 cells, treated with nicotine and benzo(a)pyrene suggested that miR-146a
