First, {TAIL ~ the labeling agent} must contain {HEAD ~ a label}; second, the labeling agent must physically or chemically bind to the binding partner or the binding agent 
First , { TAIL ~ the labeling agent } must contain { HEAD ~ a label } ; second , the labeling agent must physically or chemically bind to the binding partner or the binding agent
OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH