An ultrastructural study demonstrated that prominent dilated rER, multi-laminar rER and complex stellate arrangements of rER appeared to be morpho-genetically related in APL.22 In this study, all the giant inclusions, Auer bodies, rER, and primary granule were characterized by high electron density and activity of myeloperoxidase of APL. This evidence concerns the gene MPO and acute promyelocytic leukemia.