NPM1 (nucleophosmin or nucleolar phosphoprotein B23) is a multifunctional phosphoprotein that shuttles between the nucleus and cytoplasm and is involved in ribosomal processing and export, chromatin remodeling, DNA replication and repair, cell cycle control and molecular chaperoning. Simple variants in NPM1 are seen in about 10% of all tumors, most commonly in hematopoietic tumors.