Countering the positive aspects of immunity to tumours, the selective pressure of immune surveillance by CTLs on genetically unstable tumour populations may yield tumours that have lost expression of antigen processing machinery (APM) components, often resulting in reduced assembly of functional major histocompatibility complex (MHC) molecules that would normally bind peptide fragments derived from pathogens and tumours and display them on the cell surface for recognition by the appropriate CTL1,5–8. The gene discussed is SLC44A1; the disease is neoplasm.