I'm staining mouse lung and artery  tissue for PCNA. I'm using several types of tissue (small intestine, thymus, spleen, and liver) as a postive control for PCNA IHC. The last experiment I did, I stained the thymus for PCNA, and I added a thymus negative control (no anti-PCNA). As with the small intestine, the majority of staining was outside the nucleus. I thought it was background, but the negative control didn't show any background, and as i mentioned above, the only difference between them is the addition of the primary antibody to PCNA, so I'm assuming the antibody is recongnizing something on the tissue. So my main question is does anyone know or heard of PCNA being localized in areas of the cell other than the nucleus? Or do you suggest the thymus being a "bad" positive control? Thank you for your time. 

Nidal E Muvarak
Associate Research Specialist
Department of Biomedical Engineering
University of Wisconsin-Madison
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