Cats of group 10K and group 100K were euthanized at week 20, and tissue samples from popliteal and mesenteric lymph nodes, bone marrow, spleen, kidney, urinary bladder, lungs, thymus, myocardium, parotid gland, and pancreas were collected within 30 min post-mortem. Samples were snap-frozen in liquid nitrogen. Ten mg of tissue were used for the extraction of DNA using MagNA Pure LC DNA Isolation Kit II (Tissue kit, Roche Diagnostics). FeLV provirus was quantified by real-time PCR as described [39, 40] . To guarantee a comparable sensitivity, samples yielding less than 15 000 fGAPDH copies per reaction were re-extracted until a suitable concentration was reached. In addition, samples were collected under sterile conditions from mesenteric lymph node, urinary bladder, lungs, thymus, and myocardium for virus isolation. Tissue samples were co-cultured with FEA cells and supernatants of the co-cultures collected at days 4, 8, 12, 16, 20, 24 , and 28 post-inoculation were tested for p27 antigen by ELISA as described [7] . Bone marrow samples were cultured in medium and supernatants collected at days 18 and 21 were used for total nucleic acid isolation (MagNA Pure LC Total Nucleic Acid Isolation Kit, Roche Diagnostics) and samples were tested for the presence of FeLV RNA. Additionally, supernatants were tested by p27 ELISA [26] .