A and B Amino acid sequence of the TM and CT domains. A. Shown is a sequence alignment of the transmembrane (TM) and cytoplasmic tail (CT) domains in the hemagglutinin protein. Protein sequences from different human influenza strains belonging to the H1, B, and H3 subtypes have been included and the cysteine residues highlighted. B. The amino acid sequences of the wild-type and site directed variants of H3 A/Perth and H3 A/Texas rHA are shown. The position of the cysteine residues and the modifications are indicated. The sequences for the TM and CT domains are underlined and double underlined, respectively.