(… PAH-zih-tiv) Describes cells that have a protein called ALK on their surface. In cancer, it is also used to describe cells that have a change in the structure of the ALK gene or that make too much ALK protein. In normal cells, ALK helps control cell growth. Cancer cells that have the changed ALK gene or make too much ALK protein may grow more quickly. Knowing whether a cancer is ALK positive may help plan treatment. Cancers that may be ALK positive include neuroblastoma, non-small cell lung cancer, and anaplastic large cell lymphoma. Also called anaplastic lymphoma kinase positive.