Genetically Recombined Experimental Legionaries (GRELs) were the Earth forces' shock troops. Artificial humans, designed in gene labs and force-grown in tanks, they formed the vanguard of the invasion. All individuals within a particular GREL class are physically identical and have the same basic personalities and abilities, although environmental factors lead to a certain amount of differentiation. GRELs are sterile and were treated as little more than flesh and blood robots by their human officers.