{TAIL ~ The labeling agent} is {HEAD ~ a composition}, usually a conjugated chemical compound which has two properties 
{ TAIL ~ The labeling agent } is { HEAD ~ a composition } , usually a conjugated chemical compound which has two properties
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH