In this regard, as a monotherapy, Chk1 inhibitors might hold greater promise, given their demonstrable in vitro efficacy in quiescent as well as proliferating CLL cells [69], although in the proliferating population, ATR inhibitors acting upstream of Chk1 might produce more wide-ranging antitumor effects [10,130,131]. This evidence concerns the gene CHEK1 and B-cell chronic lymphocytic leukemia.