Until recently, the molecular identification of rare genes for monogenic disorders was laborious, often consuming many years of painstaking effort. Now, in the era of massively parallel DNA sequencing (next-generation sequencing), it is becoming routine. We cover the principles in Section 8.1. Some difficulties remain, however, because for some singlegene disorders the disease phenotypes do not have a very well-defined, distinctive pathology. And a good deal of follow-up work will be needed to dissect out all the factors in monogenic diseases, which are sometimes rather complex, as described in Section 7.7.