PCI-32765 inhibited MM stem-like cells. (A) Stem-like PRMI8226 MM cells (RPMI SP) sorted by flow cytometry were incubated with PCI-32765 in the presence or absence of BMSCs for 3 days, followed by a luminescent cell viability assay. (B) Clonogenic stem-like cells, purified from MM patient (n = 5) BMMCs by immunomagnetic depletion of CD138⁺ plasma cells and CD34⁺ normal hematopoietic progenitors, were subjected to colony formation assays in methylcellulose in the presence of PCI-32765. Btk expression in such clonogenic populations from 3 MM patients was shown by immunoblotting.