ABCA4 and cancer: Abcc5 is an orphan ATP-binding cassette transporter that is reported to export structurally unrelated compounds including various anti-cancer drugs, glutamate conjugates, such as N-acetyl-aspartyl-glutamate (NAAG), N-acetyl-aspartyl-glutamyl-glutamate (NAAG2), and beta-citryl-glutamate (BCG), as well as cyclic nucleotides and haem [4, 25–27].