Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:HA monomer, consisting of two linked subunits; HA1 (~324 amino acids) and HA2 (~222 amino acids, (Webster et al., 1992), totally weighs about 60 kDa in the unglycosylated form (Sriwilaijaroen et al., 2012).
Category: