Blood Smear of Leukemic Cells (IMAGE) University of Pennsylvania School of Medicine Caption Blood smear of leukemic cells positive for myeloperoxidase, a marker of myeloid leukemia. Blue cells are negative and brown cells are positive. Credit Warren S. Pear, M.D., Ph.D., and Karen Keeshan, Ph.D., University of Pennsylvania School of Medicine; Jon Aster, M.D., Ph.D., Brigham and Women's Hospital Usage Restrictions none License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.