A method of identifying a subject having cancer who is likely to be responsive to a treatment compound, wherein the method comprises: (a) determining the level of a biomarker in a sample from a subject who has been administered the treatment compound; and (b) diagnosing the subject as being likely to be responsive to the treatment compound if the level of the biomarker in the sample is different from a reference level of the biomarker; wherein the treatment compound is 2-(4-chlorophenyl)-N-((2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindolin-5-yl)methyl)-2,2-difluoroacetamide (Compound D) or 2-(4-fluorophenyl)-N-((2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindolin-5-yl)methyl)-2,2-difluoroacetamide (Compound E), or a stereoisomer or a mixture of stereoisomers, tautomer, pharmaceutically acceptable salt, solvate, isotopologue, hydrate, co-crystal, clathrate, or a polymorph thereof; and wherein: (i) the biomarker is an eRF3 family member selected from the group consisting of GSPT1 and GSPT2, and wherein the method comprises diagnosing the subject as being likely to be responsive to the treatment compound if the level of the biomarker is lower than the reference; (ii) the biomarker is IKZF1, and wherein the method comprises diagnosing the subject as being likely to be responsive if the level of the biomarker is lower than the reference; or (iii) the biomarker is selected from the group consisting of ATF4, ATF3, DDIT3, cleaved PARP, SRSF3 NMD transcript, and SRSF6 NMD transcript, and wherein the method comprises diagnosing the subject as likely to be responsive if the level of the biomarker is higher than the reference. (a) determining the level of a biomarker in a sample from a subject who has been administered the treatment compound; and (b) diagnosing the subject as being likely to be responsive to the treatment compound if the level of the biomarker in the sample is different from a reference level of the biomarker; wherein the treatment compound is 2-(4-chlorophenyl)-N-((2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindolin-5-yl)methyl)-2,2-difluoroacetamide (Compound D) or 2-(4-fluorophenyl)-N-((2-(2,6-dioxopiperidin-3-yl)-1-oxoisoindolin-5-yl)methyl)-2,2-difluoroacetamide (Compound E), or a stereoisomer or a mixture of stereoisomers, tautomer, pharmaceutically acceptable salt, solvate, isotopologue, hydrate, co-crystal, clathrate, or a polymorph thereof; and wherein: (i) the biomarker is an eRF3 family member selected from the group consisting of GSPT1 and GSPT2, and wherein the method comprises diagnosing the subject as being likely to be responsive to the treatment compound if the level of the biomarker is lower than the reference; (ii) the biomarker is IKZF1, and wherein the method comprises diagnosing the subject as being likely to be responsive if the level of the biomarker is lower than the reference; or (iii) the biomarker is selected from the group consisting of ATF4, ATF3, DDIT3, cleaved PARP, SRSF3 NMD transcript, and SRSF6 NMD transcript, and wherein the method comprises diagnosing the subject as likely to be responsive if the level of the biomarker is higher than the reference. (i) the biomarker is an eRF3 family member selected from the group consisting of GSPT1 and GSPT2, and wherein the method comprises diagnosing the subject as being likely to be responsive to the treatment compound if the level of the biomarker is lower than the reference; (ii) the biomarker is IKZF1, and wherein the method comprises diagnosing the subject as being likely to be responsive if the level of the biomarker is lower than the reference; or (iii) the biomarker is selected from the group consisting of ATF4, ATF3, DDIT3, cleaved PARP, SRSF3 NMD transcript, and SRSF6 NMD transcript, and wherein the method comprises diagnosing the subject as likely to be responsive if the level of the biomarker is higher than the reference.