Table 2.

Classes of MLV

X-tropic	(from the Greek xenos, meaning “foreign,” and tropos, meaning “turning”) Viruses that infect and replicate efficiently only in cells from an animal species foreign to the host
E-tropic	(from the Greek oikos, meaning “home” or “one’s environment”) Viruses that infect and replicate efficiently in cells from their own host species
A-tropic	(from the Greek amphos, meaning “both”) Viruses that infect and replicate efficiently both in cells from their own host species and in cells from heterologous species
P-tropic*	Viruses that infect and replicate efficiently both in cells from their own host species and in cells from heterologous species

^*Also known as mink cell focus-inducing virus and can be distinguished from A-tropic MLV by sequence and antigenic differences.