A human artificial chromosome (HAC) is a microchromosome that can act as a new chromosome in a population of human cells. That is, instead of 46 chromosomes, the cell could have 47 with the 47th being very small, roughly 6–10 megabases (Mb) in size instead of 50–250 Mb for natural chromosomes, and able to carry new genes introduced by human researchers. Ideally, researchers could integrate different genes that perform a variety of functions, including disease defense.