There are several alleles of the gene which creates Kell protein. Two such alleles, "K1" (Kell) and "K2" (Cellano), are the most common. The kell protein is tightly bound to a second protein, XK, by a disulfide bond. Absence of the XK protein (such as through genetic deletion or through a single point mutation within the coding region of the XK gene), leads to marked reduction of the Kell antigens on the red blood cell surface. Absence of the Kell protein (K0), however, does not affect the XK protein.