Sentence:  They may be used to <WSD>label</WSD> and image a protein within tissue, to isolate cells on the basis of marker expression, or to physically capture a protein from a complex biological mixture.... 
Meaning of the word marked with <WSD> tags:  To add a detectable substance, either transiently or permanently, to a biological substance in order to track the presence of the label-substance combination either "in situ" or "in vitro"