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Table 1 Minimal evolutionary steps to acquire all 34 pandemic markers.

From: Conserved amino acid markers from past influenza pandemic strains

Initial strain Region Shift Drift
H1N1 swine Henan/Tianjin H5, N1 199 PB2
117 NS1
H1N1 human New Zealand H9, N2 211 PB1
  Australia H7, N2 117 NS1
  U.S.A., Asia H5, N1 (one or both)
  1. First column shows the initial strain, the second column shows region where strain is found, the third column shows double reassortments taken (Shift) and column four shows the mutations (Drift) taken. The human case (row 2) involves three subtypes (H9N2, H7N2, and H5N1) and one or two mutations.