The MRP/ABCC family contains nine members (MRPs1-9, ABCCs 1-6 and ABCCs 10-12, respectively) with sizes from 1325 to 1545 amino acids. This probably completes the family, as there are no other putative MRP/ABCC genes among the 49 human ABC transporter genes. ABCC7/CFTR is a chloride channel, and as such is not considered a transporter. ABCC8/SUR1 and ABCC9/SUR2, the sulfonylurea receptors, are the ATPsensing subunits of a complex potassium channel and are not known to transport any substrates. ABCC8/SUR1 and ABCC9/SUR2 are intracellular ATP/ADP sensors, regulating the permeability of specific K-channels. Mutations in CFTR/ABCC7 cause cystic fibrosis, and mutations in the ABCC8/ SUR1 underlie a genetic disorder known as persistent hyperinsulinemic hypoglycemia of infancy. MRP10/ABCC13 is clearly a pseudo-gene which encodes a truncated protein highly expressed in fetal human live with the highest similarity to MRP2/ABCC2 but without transporting activity. MRP10/ABCC13 is considered the result of degenerative process due to deletion of exons encoding the NH2- terminal region of a progenitor relatively closely related to MRP1/ABCC1. 
These four ABCC members do not transport any substrates. The MRP/ABCCs, CFTR/ABCC7, and the SUR1 & 2 are considered to evolve from a common ancestor, and these proteins are now grouped together in the C branch of the ABC transporter family. The most striking thus far is the absence of Mrp8/ABCC11 in mice. It is important to precisely define the substrate specificities of all human MRP/ABCC members as this will provide critical information on their physiological, pharmacological, and toxicological roles. Identification of xenobiotic substrates may suggest a potential role of the MRP/ABCC transporting proteins in clinical drug resistance and in protection against a wide range of environmental toxicants. Identification of exogenous substrates of the MRP/ABCC proteins has heavily relied on an assessment of their ability to confer resistance to candidate cytotoxic drugs and xenobiotics.