The evolutionary process by which each duplicate gene or cis-acting feature loses a different, complementary part of its ancestral function, but combined, they retain the full complement of their ancestral functions. For protein coding genes, subfunctionalization can happen at the level of protein function, or can happen in cis-acting regulatory sequences (e.g. CNSs). An example would be a gene with essential functions A and B that is duplicated. One duplicate loses function A; the other loses function B. As long as they are both present, they retaining the full function (A and B) of their preduplicated state. Example of the functions A and B are protein binding and phosphorylation, or expression in root and leaves.