B-FN accumulates around the vessels of exponentially growing tumors but not on vessels of quiescent tumors. Cryostat sections of the SKMEL-28 xenograft in the exponentially growing phase (A and C) and in the quiescent phase (B and D). Sections shown in (A) and (B) were double stained using the antibody fragment L19 to B-FN (red staining) and the MoAb KI67 specific for proliferating cells (brown staining). Sections shown in (C) and (D) were double stained using the antibody fragment L19 to BFN (red staining) and a rat MoAb recognizing murine CD31, a marker of endothelial cells (brown staining). The bar corresponds to 10 μm.