CDKN1B and cancer: Figure 2D shows that NPM1 was co-immunoprecipitated with p27 in mammalian cells, demonstrating that NPM1 interacts with p27, and indeed, more strongly in cancer cells than in normal cells. Immunoprecipitation using control rabbit IgG yielded only trace amount of NPM1 and p27 proteins in the precipitates (Figure S1), indicating that NPM1 is specifically co-immunoprecipitaed with p27.