Correlation of XIAP levels with in vitro response to XIAP antagonists. Protein lysates were prepared from 21 of 27 AML samples. Samples were normalized for total protein content, and analyzed by SDS-PAGE/immunoblotting using antibodies specific for XIAP and β-actin. Immunoblot data were quantified by scanning densitometry, normalized for β-actin expression, and compared with a sample of normal peripheral blood stem cells as an internal reference control. The figure shows the relative level of XIAP protein expressed as a fold increase over the reference standard for patient samples dichotomized by their response to the XIAP inhibitor (LD₅₀ < 10 μM or LD₅₀ ≥ 10 μM). Horizontal lines represent the median XIAP protein level for the group.