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