Immunohistochemical detection of hFIX:Ag in the liver. Immunohistochemistry was performed using rabbit anti-hFIX antibody to detect hFIX:Ag in liver biopsies taken 186 days after fetal intravascular hFIX lentivirus injection into normal MF1 mice. Strong, cytoplasmic staining was seen (left) in contrast to occasional staining in vessel lumina of liver tissue from uninjected MF1 mice (right). Scale bar is 20 μm.