The grade of a cancer reflects how abnormal it looks under the microscope. There are several grading systems for cancer, such as the Gleason score for prostate cancer. Each grading system divides cancer into those with the greatest abnormality (poorly differentiated), the least abnormality (well-differentiated), and those in between (moderately differentiated). Grading is done by the pathologist who examines the tissue from the biopsy. It is important because higher grade cancers tend to grow and spread more quickly and have a worse prognosis.