The invention relates to a mammalian cell, particularly a human cell, expressing a first isoform of a surface protein, wherein the first isoform is functionally indistinguishable, but immunologically distinguishable from a second isoform, for use in a medical treatment of a patient having cells expressing the second isoform form of the surface protein. The invention further relates to an agent selected from 1) a compound comprising, or consisting of, an antibody or antibody-like molecule and 2) an immune effector cell bearing an antibody-like molecule or an immune effector cell bearing a chimeric antigen receptor, for use in a method of treatment of a medical condition, wherein the agent is specifically reactive to either a first or a second isoform of a surface protein, wherein the first isoform is functionally indistinguishable, but immunologically distinguishable from the second isoform, and wherein the agent is administered to ablate a cell bearing the isoform that the agent is reactive to.