I'm gonna do IF in old mouse frozen sections to detect the pP53 and P21 level. The tissues were fixed by 4%PFA at RT for 30 min, 20um thick. The 2 antibodies are made from mouse, whcih caused the background signal is strong and there seemed no positive cell detected. I tested the antibodies in cell line by adding etoposide( to induce senescence).Both of them worked well. So any suggestions to help stain in the frozen sections? Many thanks!