{HEAD ~ The cells} contain {TAIL ~ an effector gene} under control of a transcriptional control element that causes the gene to be expressed in relatively undifferentiated cells in the population 
contain
{ HEAD ~ The cells } contain { TAIL ~ an effector gene } under control of a transcriptional control element that causes the gene to be expressed in relatively undifferentiated cells in the population
HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH