Most mutations induced by ENU mutagenesis involve single-base changes, which are also, along with small indels and copy number variations, the most common type of natural variation in humans and other mammals, where few regulatory mutations have yet been identified. Whereas a nonsynonymous mutation in a protein-coding sequence can have severe effects on the structure and function of the protein, many regulatory sequences have loose consensus sequences, and variations in them, as noted already, may have subtle effects and go unnoticed, especially in superficial phenotypic screens.
