A host cell expressing a mutant TDP-43 protein which has the activity of forming intracellular inclusions, wherein the cell comprises a promoter operably linked to a mutant TDP-43 nucleotide sequence and wherein the mutant TDP-43 protein is either: (i) any one of the following proteins (a) to (c): (a) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 187-192; (b) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 78-84 and amino acids 187-192; and (c) a protein that has an amino acid sequence having 1 to 10 amino acids deleted from, substituted in or added to the amino acid sequence (a) or (b); or (ii) any one of the following proteins (a) to (d): (a) a protein whose amino acid sequence consists of amino acids 162-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (b) a protein whose amino acid sequence consists of amino acids 218-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (c) a protein whose amino acid sequence consists of amino acids 1-161 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; and (d) a protein whose amino acid sequence consists of an amino acid sequence having 1 to 10 amino acids deleted from or added to the amino acid sequence (a), (b) or (c). (i) any one of the following proteins (a) to (c): (a) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 187-192; (b) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 78-84 and amino acids 187-192; and (c) a protein that has an amino acid sequence having 1 to 10 amino acids deleted from, substituted in or added to the amino acid sequence (a) or (b); or (a) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 187-192; (b) a protein having the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2 lacking amino acids 78-84 and amino acids 187-192; and (c) a protein that has an amino acid sequence having 1 to 10 amino acids deleted from, substituted in or added to the amino acid sequence (a) or (b); or (ii) any one of the following proteins (a) to (d): (a) a protein whose amino acid sequence consists of amino acids 162-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (b) a protein whose amino acid sequence consists of amino acids 218-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (c) a protein whose amino acid sequence consists of amino acids 1-161 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; and (d) a protein whose amino acid sequence consists of an amino acid sequence having 1 to 10 amino acids deleted from or added to the amino acid sequence (a), (b) or (c). (a) a protein whose amino acid sequence consists of amino acids 162-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (b) a protein whose amino acid sequence consists of amino acids 218-414 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; (c) a protein whose amino acid sequence consists of amino acids 1-161 of the amino acid sequence of wild-type TDP-43 shown in SEQ ID NO:2; and (d) a protein whose amino acid sequence consists of an amino acid sequence having 1 to 10 amino acids deleted from or added to the amino acid sequence (a), (b) or (c).