B HA and NP represent the Cal/7/9 wildtype proteins. Chimeric HA (cHA) 1 contains the head regions of H2 HA, cHA2 of H10 HA and cHA3 of H13 HA, while they all contain the Cal/7/9 HA-stalk region. Headless HA (HL) contains a deletion in the HA-head region (dashed line). Modified headless HA (mHL1 and mHL2) contain stabilizing mutations (black boxes) and lack additional internal parts. All constructs were codon-optimized and carry a V5-tag at their C-terminus. 