(A) Shown is the IFITM3 promoter region and open reading frame. The G (protective) allele promotes methylation of an adjacent nucleotide, while the A (risk) allele ablates methylation. Methylation blocks binding of the CTCF transcriptional repressor, allowing transcription to occur, while in the absence of methylation, CTCF binds to the IFITM3 promoter and represses IFITM3 transcription. A key is shown at the bottom of the panel. (B) IFITM3 promoter methylation promotes IFITM3 protein expression in memory CTL, which protects CTL survival after influenza virus challenge. CTL, CD8+ T lymphocyte.