Table 1. Amino acids of the HA1 sequence involved in changes of charge state when comparing 1968, 1985 and 2000.

20001968	Positive (+)	Neutral	Negative (−)
Positive (+)	—	57/156	—
Neutral	142/145/155/158/160/173/276	189/197	53/62/83/188
Negative (−)	2/82	31/63/275	172

‘Group 2’ amino acids, as defined in the main text, are shown in italics on the diagonal – all three exhibited an increase in charge of +1 in 1985, followed by a reversion before 2000. All other amino acids belong to ‘group 1’. Note that those elements below the diagonal contribute to a positive overall charge, and vice versa for those above the diagonal.