The present invention provides a novel substance that stably binds to a
surface protein expressed on NK cells. The present invention provides a
monobody that binds to a
surface protein of an NK
cell, wherein the
monobody has an
amino acid sequence of any one of (1) to (3): (1) an
amino acid sequence represented by any one of SEQ ID NOs: A1 to A12; (2) an
amino acid sequence given by deletion, substitution, and / or addition of one or more amino acids in an amino acid sequence represented by any one of SEQ ID NOs: A1 to A12 excluding arbitrary amino acids; and (3) an amino acid sequence having an identity of 80% or more with an amino acid sequence represented by any one of SEQ ID NOs: A1 to A12 excluding arbitrary amino acid sequences, wherein X1, X2, X3, X4, X5, and X6 in SEQ ID NOs: A1 to A12 each independently represent an arbitrary amino acid, and n1, n2, n3, n4, n5, and n6 each independently represent an arbitrary natural number.X1n1 X2n2 X3n3 (SEQ ID NO: A1)PIWGRYGTTVRGQTYYWWMLX1n1X2n2 X3n3 (SEQ ID NO: A2)WTWLGYEYYWWDSKPGAIGIX1n1 X2n2 X3n3 (SEQ ID NO: A3)TYWDWGFRWSWARRTHISWFX4n4X5n5 X6n6 (SEQ ID NO: A4)PGYYMWQRGVINGQERAYGHPEHFX4n4X5n5 X6n6 (SEQ ID NO: A5)RYKYHRRINGTQWYVFNEAGDVAGYX1n1X2n2 X3n3 (SEQ ID NO: A6)GPIVSYSWWYHWVGYVNEKYDAWQSYVQX1n1X2n2 X3n3 (SEQ ID NO: A7)QAYWFEYTWWLEVVYDDQWTSGX4n4X5n5 X6n6 (SEQ ID NO: A8)TSPWYNGYDQYKSWNLEEFHX4n4X5n5 X6n6 (SEQ ID NO: A9)TPPWIHKGWVTPYGRKWVGMNGWX1n1X2n2 X3n3 (SEQ ID NO: A10)LWHYYGDQYWWMYVPEYYFNPWX1n1X2n2 X3n3 (SEQ ID NO: A11)TDRNYDITYGMQYWWDGYSYSEEAWYYX4n4X5n5 X6n6 (SEQ ID NO: A12)WSWESYWHGTYTFMKDIPSYY