Nucleophosmin (NPM) / B23 / numatrin is a phosphoprotein associated with cell nucleoli. Treatment of cells with cytotoxic drugs may make nucleophosmin enter nucleoplasm. It is reported that nucleophosmin shuttles continuously between cytoplasm and nucleoli to carry newly synthesized ribosomal proteins into nucleoli. NPM- ALK is a hybrid of the anaplastic lymphoma kinase (ALK) gene and the nucleophosmin (NPM) gene resulting from the t(2;5)(p23;q35) translocation found in anaplastic large cell lymphoma. In acute promyelocytic leukemia NPM gene is fused to RARalpha gene in (5;17) (q33;q12) transition.