Expression of MCL-1 in neoplastic human mast cells. (A) Immunohistochemical detection of tryptase (left) and MCL-1 (right) in neoplastic MCs in a patient with indolent SM (ISM). Adjacent bone marrow sections were incubated with antibodies against tryptase or MCL-1. Immunohistochemistry was performed as described. (B) Immunocytochemical detection of MCL-1 in HMC-1.2 cells exhibiting the KIT mutation D816V (left). Immunocytochemistry was performed using a polyclonal anti–MCL-1 antibody. Preincubation of the antibody with a specific blocking peptide resulted in a negative stain (right). Similar staining results were obtained with HMC-1.1 cells lacking KIT D816V (not shown). (C) Northern blot analysis of HMC-1.1 cells and HMC-1.2 cells using an MCL-1–specific cDNA probe. β-Actin loading control is also shown. (D) RT-PCR analysis of MCL-1 mRNA expression in K562 cells, HMC-1.1 cells, HMC-1.2 cells, and purified (purity greater than 98%) neoplastic human mast cells obtained from one patient with mast cell sarcoma (MCS) (patient 1) and 2 patients (patients 2 and 3) with mast cell leukemia (MCL). The PCR reaction was controlled by omitting the RT step (−RT).