Auer rods are red/pink colored, rod-shaped cytoplasmic inclusions that occur only in cells of the myeloid lineage. They arise from aggregation of lysosomal proteins (specifically, myeloperoxidase and acid phosphatase). Auer rods are most commonly associated with AML and APL, but may also occur in MDS or CMML. Auer rods are invariably malignant.