ASS1 is not expressed in most AML. (A) The 2 enzymes required for endogenous arginine synthesis are shown together with the products of arginine following ADI-PEG 20 therapy. (B) ASS1 expression was assessed in bone marrow using immunohistochemistry: positive control, liver (top left); control bone marrow (top right). The rest of the panels are pictures of trephines with the diagnosis indicated on each panel. The 2 AML samples are negative (middle right, sample T19; middle left, sample T22), whereas the 2 APL samples express ASS1 (bottom right, sample T3; bottom left, sample T2). Slides were analyzed using a Leica DM2500 microscope with ×100 magnification, and images were acquired by a Leica DFC320 camera and processed using Leica IM50 software. (C) The expression of ASS1 and ASL in primary AML lysates was tested using western blotting. Results for 7 AML, 1 APL, and 1 granulocyte–colony-stimulating factor mobilized peripheral blood (GMPB) samples are shown. (D) The relative expression of ASS1 was quantified by real-time PCR in 4 APL, 38 AML, and 11 GMPB samples. Relative expression of ASS1 was greater in APL than AML or GMPB. *P < .001. (E) Methylation of the ASS1 gene was assessed in 7 primary AML, 1 APL, and 1 GMPB samples. Six of 7 AML samples show some methylation, whereas the APL does not. U, unmethylated DNA; M methylated DNA.