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(a) Kernel density estimation for the protein distance map of 2009 A(H1N1) influenza as of 5 December 2009. (b) The protein distance map of 2009 A(H1N1) influenza. The vertical and horizontal axes of both figures represent protein distance as defined in Equation (1). A 0.0018 unit of protein distance equals one substitution in the HA protein sequence of H1N1. The height and colors in (a) both represent the density of isolates.
