The virus:
The main envelope protein is gp70, and it determines subgroup and host range in-vitro. Three subgroups of FeLV exist and their classification is based upon envelope interference and neutralization in cell culture. Interference is complex but it means that A will block all its receptors such that B but not A can reinfect the cell culture.
All isolates of FeLV contain A. Fifty percent contain B and 1% contain C. All subgroups grow in cat cell cultures without cpe.
Subgroup B is a recombinant between transmissible FeLV-A and endogenous B envelope genes which are present in all cat cells. Subgroup C isolates are mutants of subgroup A.
Subgroups B and C, but not A, grow in human cells in-vitro. This caused a hugh panic when Glasgow vet school discovered it; but people, even vets or those in leukaemic clusters, have no antibodies to FeLV or virus in their blood.