ASS1-expressing normal hematopoietic cells are seen following suc-cessful AML clearance by ADI-PEG 20. Bone marrow sections from mice transplanted with AML sample 3 following treatment with vehicle or ADI-PEG 20 were stained with hematoxylin and eosin to assess residual hematopoiesis. The majority of cells in mice receiving vehicle were leukemic (A), whereas normal hematopoietic cells, including megakaryocytes (arrowed), are seen in mice treated with ADI-PEG 20 (B). ASS1 expression was lower in mice receiving vehicle (C) than mice receiving ADI-PEG 20 (D). Slides were analyzed using a Leica DM2500 microscope (magnification ×400 [A-B], ×200 [C-D]), and images were acquired by a Leica DFC320 camera and processed using Leica IM50 software. ASS1 was not expressed by AML cells (E) but was expressed by nonhuman marrow cells in mice treated with ADI-PEG 20 (F). Images were captured on an LSM510 Meta confocal laser microscope using a 40 × 1.3 oil immersion objective.