The present invention provides an antigen binding protein specifically binding to a CT45 antigenic peptide that is in a complex with a major histocompatibility complex (MHC) protein, wherein the CT45 antigenic peptide comprises or consists of the amino acid sequence of SEQ ID NO: 138 (KIFEMLEGV) and wherein the antigen binding protein comprises a first polypeptide comprising a variable domain VA comprising complementarity determining regions CDRa1, CDRa2 and CDRa3 and a second polypeptide comprising a variable domain VB comprising CDRb1, CDRb2 and CDRb3. Also provided are nucleic acids encoding the antigen binding proteins, vectors comprising the nucleic acids, recombinant cells expressing the antigen binding proteins and pharmaceutical compositions comprising the antigen binding proteins. The invention further provides the antigen binding proteins for use in medicine and a method of producing the antigen binding protein.