Poly(ADP-ribose) polymerase (PARP)-1 expression in joint tissue. (a) Western blot analysis of PARP-1 and actin proteins in fibroblast-like synoviocyte extracts of the indicated genotypes; 100 ng of purified PARP-1 was the control. Immunostaining for PARP-1 expression in the knee joint sections from (b) parp-1+and (d) parp-1o/o mice. Staining for nuclear PARP-1 showed clear immunopositivity on chondrocytes from parp-1+ mice (arrows) whereas chondrocytes from parp-1o/o (arrowheads) remained negative. Negative control staining, by omitting the primary antibody, in knee joints from (c) parp-1+ and (e) parp-1o/o mice.