– WEHI-539 is a highly potent and selective BCL-XL inhibitor. The prosurvival BCL-2 family protein BCL-X(L) is often overexpressed in solid tumors and renders malignant tumor cells resistant to anticancer therapeutics. WEHI-539 has high affinity (subnanomolar) and selectivity for BCL-X(L) and potently kills cells by selectively antagonizing its prosurvival activity. WEHI-539 will be an invaluable tool for distinguishing the roles of BCL-X(L) from those of its prosurvival relatives, both in normal cells and notably in malignant tumor cells, many of which may prove to rely upon BCL-X(L) for their sustained.