Feline leukemia virus (FeLV) is an RNA retrovirus that is a transmitted both vertically (mother to fetus) and horizontally. Envelope glycoprotein (gp70) defines the FeLV antigenic subgroup as A, B, C, or combinations of these; determines infectivity, host range, and pathogenicity; and elicits the protective host neutralizing-antibody response that occurs with natural exposure or vaccination. Only subgroup A is infectious.