We demonstrated that (i) peripheral blood MCL cells express low level of Bcl-xL and are highly sensitive to ABT-199, (ii) upon CD40 stimulation there is an increase in Bcl-xL protein level but not Mcl-1 or Bcl-2, and (iii) that silencing of BCL-xL overcomes ABT-199 resistance induced by CD40 stimulation. This evidence concerns the gene MCL1 and mantle cell lymphoma.