Amino acids crucial for lipid membrane order/cholesterol binding are addressed. Influenza virus HA transmembrane domain (TMD): α-helical conformation. Paramyxovirus F TMD: β-strand rich conformation that promotes fusion. Influenza HA and paramyxovirus F TMD trimeric self-association is discussed. S-acylation (highly conserved in HA and occasional in F) is discussed in relation to fusion.