Donor sites are divided into five types: single functional constitutive donor SD, alternative donor for cassette exons CD and multiple functional donors MD1, MD2, and MDN. MD1 is the left most functional donor, MD2 is the donor immediately downstream of MD1, and MDN represents additional downstream donors. The classification scheme similarly extends to acceptor sites: SA (single constitutive acceptor), CA (acceptor for cassette exon), MA1 (first multiple acceptor), MA2 (second multiple acceptor), and MAN (multiple acceptors greater than 2).