Constitutive activation of MEK1/2, but not of AKT in p-ERK⁺ CLL cells. (A) Extracts from Daudi cells either untreated or stimulated with anti-IgM, and from purified CLL cells obtained from the peripheral blood of 8 representative CLL patients previously tested positive for p-ERK1/2 (p-ERK⁺) or negative for p-ERK1/2 (p-ERK⁻), were examined for activation of AKT by immunoblot with an antibody specific for phospho-AKT (Ser473). Blots were stripped and reblotted with AKT-specific antibody (bottom panel). A vertical line has been inserted to indicate a repositioned gel lane. (B) Extracts from purified CLL cells isolated from 4 p-ERK1/2⁻ and 4 p-ERK1/2⁺ CLL patients were examined for MEK phosphorylation by Western blot with an antibody specific for p-MEK1/2 (top panel). Total MEK protein levels were examined on the same blot using an antibody specific for total MEK1/2 (bottom panel).