Identification of ADIR as the polymorphic gene responsible for RDR2 recognition. BLAST searching of SVAPALALFPA against a translated EMBL database revealed 100% identity to amino acids 13 to 23 from an alternative ORF of the ADIR gene. A known SNP at nucleotide 78 results in an amino acid change from S>F. (A). Both patient-type peptide SVAPALAL-F-PA (▪) and donor-type peptide SVAPALAL-S-PA (□) were synthesized and tested for RDR2 reactivity on T2 cells in a ⁵¹Cr release assay. Only cells loaded with the patient-type peptide but not cells loaded with the donor-type peptide were lysed (B). Constructs containing patient-derived DNA were generated. The start of each construct was varied to obtain translation at the start of the transcript, resulting in both the normal and the alternative ORF or specific translation of only the normal or alternative ORF. Constructs were transiently transfected into Hela/A2 cells. RDR2 was cocultured for 24 hours, and IFN-γ release in supernatants was measured by ELISA. RDR2 stimulation was observed in all cases. Stimulation by constructs containing only the normal ORF start codon and lacking the alternative ORF start codon showed strongly diminished CTL recognition (C). Similar constructs containing the donor-derived DNA were not recognized by RDR2 (data not shown).