Abstract: An object of the present invention is to improve the efficiency of a method for producing chimeric antigen receptor (CAR)-expressing cells. The present invention provides a method for producing genetically modified mammalian cells, comprising the steps of: a) introducing a polynucleotide encoding a chimeric antigen receptor (CAR) protein to a cell population comprising T cells derived from a mammal by a transposon method to obtain a genetically modified cell population; b) providing an endogenous cell population derived from the mammal expressing a protein that binds to the CAR; and c) coculturing the genetically modified cell population of the step a) and the endogenous cell population of the step b).

Abstract: It is intended to produce a cell expressing a mutant chimeric antigen receptor (CAR) having excellent cytotoxicity to target cells. The present invention provides a genetically modified cell having introduced thereinto a polynucleotide encoding a chimeric antigen receptor (CAR) protein having a target binding domain that specifically binds to a human granulocyte-macrophage colony stimulating factor (GM-CSF) receptor, a transmembrane domain, and an intracellular signaling domain, wherein the tar get binding domain is a mutant having the substitution of glutamic acid at position 21 in the amino acid sequence shown in SEQ ID NO: 1 with another amino acid.