Indeed, a recent study of over 1,300 mouse ncRNAs showed that almost half of them exhibit precise expression patterns in different parts of the brain, including different subregions of the hippocampus, olfactory bulb, neocortex, and cerebellum [52]. While many ncRNAs, like other regulatory sequences (see below), appear to be evolving quickly, including those with well-validated functions [26], most retain conserved patches within them [26],[32], and some may be under positive selection, especially in the brain [57]. At least some of these RNAs, including intronic RNAs, are precursors for small regulatory RNAs like microRNAs (miRNAs), small interfering RNAs (siRNAs), piwi-interacting RNAs (piRNAs), and small nucleolar RNAs (snoRNAs) [58]–[68]. Many miRNAs are highly conserved from nematodes to humans, whereas others are primate-specific, and their full repertoire remains to be determined [4],[69],[70].
