The medium may be used for serum free culture or supplemented with {HEAD ~ low levels} of {TAIL ~ serum} 
of
The medium may be used for serum free culture or supplemented with { HEAD ~ low levels } of { TAIL ~ serum }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL