A multimeric ligand for use in a method for the treatment of cancer, said method comprising controlling survival of therapeutic cells in a subject, wherein the method is characterized by a) transplanting therapeutic cells that comprise a nucleic acid that encodes a chimeric polypeptide comprising a multimerization region and a caspase-9 polypeptide comprising the amino acid sequence of SEQ ID NO: 9 or a modified caspase-9 polypeptide into the subject, wherein the modified caspase-9 polypeptide comprises (i) an amino acid sequence having at least 90% sequence identity to SEQ ID NO: 9, and (ii) at least one amino acid substitution selected from the group consisting of S144A, S144D, Y153A, Y153F, S183A, S195A, S196A, S196D, S307A, D315A, A316G, T317A, T317C, T317E, T317S, P318A, F319A, F319W, F326K, D327G, D327K, D327R, Q328K, Q328R, L329E, L329G, L329K, D330A, D330E, D330G, D330N, D330S, D330V, A331K, C403A, C403S, C403T, F404T, F404W, F404Y, N405A, N405F, N405Q, N405T, F406A, F406T, F406W, and F406Y; b) administering said multimeric ligand subsequently to said subject in an amount effective to kill less than 50% of transplanted therapeutic cells that express the caspase-9 polypeptide, wherein a therapeutically effective level of therapeutic cells remains active in the subject following administration of the multimeric ligand; or administering said multimeric ligand subsequently to said subject in an amount effective to kill less than 50% of transplanted therapeutic cells that express the modified caspase-9 polypeptide, wherein a therapeutically effective level of therapeutic cells remains active in the subject following administration of the multimeric ligand; and wherein said multimeric ligand binds to the multimerization region; and said modified caspase-9 polypeptide has an elongated dose response curve in response to the multimeric ligand compared to a caspase-9 polypeptide that is not modified. a) transplanting therapeutic cells that comprise a nucleic acid that encodes a chimeric polypeptide comprising a multimerization region and a caspase-9 polypeptide comprising the amino acid sequence of SEQ ID NO: 9 or a modified caspase-9 polypeptide into the subject, wherein the modified caspase-9 polypeptide comprises (i) an amino acid sequence having at least 90% sequence identity to SEQ ID NO: 9, and (ii) at least one amino acid substitution selected from the group consisting of S144A, S144D, Y153A, Y153F, S183A, S195A, S196A, S196D, S307A, D315A, A316G, T317A, T317C, T317E, T317S, P318A, F319A, F319W, F326K, D327G, D327K, D327R, Q328K, Q328R, L329E, L329G, L329K, D330A, D330E, D330G, D330N, D330S, D330V, A331K, C403A, C403S, C403T, F404T, F404W, F404Y, N405A, N405F, N405Q, N405T, F406A, F406T, F406W, and F406Y; (i) an amino acid sequence having at least 90% sequence identity to SEQ ID NO: 9, and (ii) at least one amino acid substitution selected from the group consisting of S144A, S144D, Y153A, Y153F, S183A, S195A, S196A, S196D, S307A, D315A, A316G, T317A, T317C, T317E, T317S, P318A, F319A, F319W, F326K, D327G, D327K, D327R, Q328K, Q328R, L329E, L329G, L329K, D330A, D330E, D330G, D330N, D330S, D330V, A331K, C403A, C403S, C403T, F404T, F404W, F404Y, N405A, N405F, N405Q, N405T, F406A, F406T, F406W, and F406Y; b) administering said multimeric ligand subsequently to said subject in an amount effective to kill less than 50% of transplanted therapeutic cells that express the caspase-9 polypeptide, wherein a therapeutically effective level of therapeutic cells remains active in the subject following administration of the multimeric ligand; or administering said multimeric ligand subsequently to said subject in an amount effective to kill less than 50% of transplanted therapeutic cells that express the modified caspase-9 polypeptide, wherein a therapeutically effective level of therapeutic cells remains active in the subject following administration of the multimeric ligand; and wherein said multimeric ligand binds to the multimerization region; and said modified caspase-9 polypeptide has an elongated dose response curve in response to the multimeric ligand compared to a caspase-9 polypeptide that is not modified.