There has been considerable interest in the role of TLR4 in regulating RSV infection since RSV F protein has been reported to be a ligand for TLR4.23 A clinical study found increased TLR4 expression in blood monocytes of infants with RSV bronchiolitis,24 and a genetic susceptibility study identified two common TLR4 gene mutations which are associated with an increased risk of severe RSV bronchiolitis compared with mild disease (odds ratio 5.1 (95% confidence interval (CI) 1.4 to 18.1) and 4.0 (95% CI 1.3 to 12.5).25 This evidence concerns the gene TLR4 and respiratory syncytial virus bronchiolitis.