The authors present a detailed search for the genetic basis of variation in ambient temperature sensitivity for flowering time in Arabidopsis. Focusing specifically on the control of the expression of FLM which has been previously linked to temperature sensitivity, the authors draw on a range of modern and innovative tools to pinpoint alleles conferring variation FLM expression within the species. They identify the major alleles controlling both basal expression and temperature sensitivity of FLM segregating at high frequency in the species and confirm effects of these alleles using transgenics. Overall, this study is important in its demonstration of how modern tools in Arabidopsis can be used to rapidly identify the specific base-pairs underlying natural variation in important traits. While I think that the major conclusions seem robust and well-supported, this is a very complicated paper and I do have concerns about specific analyses and interpretations.