its deuterated counterpart. At the same time Turin and coworkers reported that human volunteers were able to distinguish cyclopentadecanone from its fully deuterated analog. To account for the different results seen with acetophenone and cyclopentadecanone, Turin and coworkers assert that "there must be many C-H bonds before they are detectable by smell. In contrast to acetophenone which contains only 8 hydrogens, cyclopentadecanone has 28. This results in more than 3 times the number of vibrational modes involving hydrogens than in acetophenone, and this is likely essential for detecting the difference between isotopomers." Turin and coworkers provide no quantum mechanical justification for this latter assertion. Vosshall, in commenting on Turin's work, notes that "the olfactory membranes are loaded with enzymes that can metabolise odorants, changing their chemical identity and perceived odour. Deuterated molecules would be poor substrates for such enzymes, leading to a chemical difference in what the subjects are testing. Ultimately, any attempt to prove the vibrational theory of olfaction should concentrate on actual mechanisms at the level of the receptor, not on indirect psychophysical testing." Richard Axel co-recipient of the 2004 Nobel prize for physiology for his work on olfaction, expresses a similar sentiment, indicating that Turin's work "would not resolve the debate – only a microscopic look at the receptors in the nose would finally show what is at work. Until somebody really sits down and seriously addresses the mechanism and not inferences from the mechanism... it doesn't seem a useful endeavour to use behavioural responses as an argument". In response to the 2013 paper on cyclopentadecanone, Block et al. report that the human musk-recognizing receptor, OR5AN1, identified using a heterologous olfactory receptor expression system and robustly responding to cyclopentadecanone and muscone (which has 30 hydrogens), fails to distinguish isotopomers of these compounds in vitro. Furthermore, the mouse (methylthio)methanethiol-recognizing receptor, MOR244-3, as well as other selected human and mouse olfactory receptors, responded similarly to normal, deuterated, and carbon-13 isotopomers of their respective ligands, paralleling results found with the musk receptor OR5AN1. Based on these findings, the authors conclude that the proposed vibration theory does not apply to the human musk receptor OR5AN1, mouse thiol receptor MOR244-3, or other olfactory receptors examined. Additionally, theoretical analysis by the authors shows that the proposed electron transfer mechanism of the vibrational frequencies of odorants could be easily suppressed by quantum effects of nonodorant molecular vibrational modes. The authors conclude: "These and other