Addressing the questions of where and when to measure epigenetic modification is considerably more complex. In the ideal scenario, we would obtain repeated measures of epigenetic modifications from samples from any region of interest in the live brain. However, given the obvious limitations to sampling live brains among the general population, we are limited at this time to either measuring brain tissue after death, or peripheral tissue in live subjects, both of which have important drawbacks, as we describe above. There is, therefore, a direct trade-off between measurement of the appropriate where and the appropriate when.
