A widely used method for classifying the cellular differentiation of cancerous tissue. The less the cancerous cells appear like normal cells, the more malignant the cancer. Two grades of 1-5, identifying the two most common degrees of differentiation present in the examined tissue sample, are added together to produce the Gleason score. High numbers indicate greater differentiation and more aggressive cancer. The grading system is name after its originator, Donald Gleason, M.D.