A pan-BCL2 inhibitor for use in a method for treating a subject with acute myeloid leukemia, acute lymphocytic leukemia, chronic lymphocytic leukemia or chronic myeloid leukemia wherein the subject has quiescent cancer stem cells in the bone marrow, wherein the subject has been identified by: (i) detecting and quantifying in at least one or a plurality of Leukemia Stem cells (LSCs) that has/have been provided in a sample from the subject, wherein the plurality of LSCs is from the bone marrow: (a) one or more B-cell lymphoma-2 family protein(s) or proteins isoform(s), wherein the BCL2 family protein or isoform is selected from the group consisting of BCL2, myeloid cell leukemia sequence 1, a B-cell lymphoma-extra large (Bcl-XL) protein, or a BFI1 protein or a pro-apoptotic BCL2 family protein, and/or a transcript encoding one or more BCL2 family protein(s) or protein isoform(s), wherein the BCL2 family transcript is selected from the group consisting of a BCL2 transcript, a myeloid cell leukemia sequence 1 transcript, a B-cell lymphoma-extra large (Bcl-XL) transcript, or a BFI1 transcript or a transcript of a pro-apoptotic BCL2 family protein; and (b) the cell cycle status or the amount of quiescence, of the LSC; and (ii) comparing (a) the quantified levels of protein or transcript in the LSC to a cell having a phenotype of a normal bone marrow stem cell, or the amount of one protein isoform to a second protein isoform of the same protein, or the amount of transcript encoding one protein isoform to the amount of transcript encoding a second protein isoform of the same protein, and (b) the cell cycle status or the amount of quiescence of the LSC to a normal bone marrow stem cell; wherein the sample from the subject has a combination of (a) increased amount of quiescence, in the LSC as compared to a a normal bone marrow stem cell, and (b) more amount of a longer splice form of a BCL2 family protein transcript as compared to a shorter alternative splice isoform of the BCL2 family transcript and/or more amount of a protein encoded by a longer splice form of a BCL2 family protein transcript as compared to the amount of a protein encoded by a shorter alternative splice isoform of the BCL2 family transcript. (i) detecting and quantifying in at least one or a plurality of Leukemia Stem cells (LSCs) that has/have been provided in a sample from the subject, wherein the plurality of LSCs is from the bone marrow: (a) one or more B-cell lymphoma-2 family protein(s) or proteins isoform(s), wherein the BCL2 family protein or isoform is selected from the group consisting of BCL2, myeloid cell leukemia sequence 1, a B-cell lymphoma-extra large (Bcl-XL) protein, or a BFI1 protein or a pro-apoptotic BCL2 family protein, and/or a transcript encoding one or more BCL2 family protein(s) or protein isoform(s), wherein the BCL2 family transcript is selected from the group consisting of a BCL2 transcript, a myeloid cell leukemia sequence 1 transcript, a B-cell lymphoma-extra large (Bcl-XL) transcript, or a BFI1 transcript or a transcript of a pro-apoptotic BCL2 family protein; and (b) the cell cycle status or the amount of quiescence, of the LSC; and (a) one or more B-cell lymphoma-2 family protein(s) or proteins isoform(s), wherein the BCL2 family protein or isoform is selected from the group consisting of BCL2, myeloid cell leukemia sequence 1, a B-cell lymphoma-extra large (Bcl-XL) protein, or a BFI1 protein or a pro-apoptotic BCL2 family protein, and/or a transcript encoding one or more BCL2 family protein(s) or protein isoform(s), wherein the BCL2 family transcript is selected from the group consisting of a BCL2 transcript, a myeloid cell leukemia sequence 1 transcript, a B-cell lymphoma-extra large (Bcl-XL) transcript, or a BFI1 transcript or a transcript of a pro-apoptotic BCL2 family protein; and (b) the cell cycle status or the amount of quiescence, of the LSC; and (ii) comparing (a) the quantified levels of protein or transcript in the LSC to a cell having a phenotype of a normal bone marrow stem cell, or the amount of one protein isoform to a second protein isoform of the same protein, or the amount of transcript encoding one protein isoform to the amount of transcript encoding a second protein isoform of the same protein, and (b) the cell cycle status or the amount of quiescence of the LSC to a normal bone marrow stem cell; (a) the quantified levels of protein or transcript in the LSC to a cell having a phenotype of a normal bone marrow stem cell, or the amount of one protein isoform to a second protein isoform of the same protein, or the amount of transcript encoding one protein isoform to the amount of transcript encoding a second protein isoform of the same protein, and (b) the cell cycle status or the amount of quiescence of the LSC to a normal bone marrow stem cell; (a) increased amount of quiescence, in the LSC as compared to a a normal bone marrow stem cell, and (b) more amount of a longer splice form of a BCL2 family protein transcript as compared to a shorter alternative splice isoform of the BCL2 family transcript and/or more amount of a protein encoded by a longer splice form of a BCL2 family protein transcript as compared to the amount of a protein encoded by a shorter alternative splice isoform of the BCL2 family transcript.