We analysed splenocyte expression of immunological genes (TLR2, TLR4, TLR9, TNF-a, TGF-b1, IL-10, FoxP3) in a time series of A. sylvaticus at Cotgrave Forest, UK from May to December 2008. Preliminary analyses of individual variation indicated that a small proportion of individuals were associated with very extreme expression values for most or all of the measurements taken (Figure 1 ). In order to categorize this variation we initially identified outlier values in log-transformed, standardized data as occurring above the third data quartile cutoff by more than 1.56 the interquartile range. Individuals with .10 outlier readings from the 30 variables measured were considered as an ''extreme expression'' group ( Figure 2) . The individual composition of this group was broadly supported by HCA, PCA and PCO for the subset of 22 expression variables without missing data values (Figure 3) .