Subsequently, two array-based platforms were developed which allowed a much broader investigation of differential methylation patterns across the genome: the Illumina 27k and 450k platforms. With the development of these platforms, several loci have emerged as robust indicators of smoking. The first consistent locus to emerge, coagulation factor II (thrombin) receptor-like 3 (F2RL3), was covered by both the 27k and 450k platforms, whereas the second, the aryl hydrocarbon receptor repressor (AHRR), was only covered by the 450k platform but has been more consistently replicated. Several other loci have been replicated in six or more studies as well, including the 2q37.1 region, 6p21.33 region, growth factor independent 1 transcription repressor (GFI1), and myosin IG (MYO1G). However, as we will demonstrate below, the AHRR locus fulfills the greatest number of criteria as specified above as indicators of potential for a robust and flexible epigenetic biomarker of smoking. In total, this review identified seven studies using the 27k platform and a further 23 using the 450k platform. Based on the results of these studies, nine more in-depth investigations of promising loci were identified
