sentence: Reactivity against three different in-house produced viral protein variants was used to differentiate between positive and negative samples : Spike trimers comprising the prefusion-stabilized spike glycoprotein ectodomain ( 28 ) ( expressed in HEK and purified using a C-terminal Strep II tag ) , Spike S1 subunit ( expressed in CHO and purified with HPC4 tag ) , and the Nucleocapsid protein ( expressed in E. coli and purified using an N-terminal His-tag ) .