PURPOSE: To provide a new gene which has a specific base sequence, codes the L-chain V region of the monoclonal antibody inhibiting the M subunit but not the M' subunit of creatinine kinase(CK) and gives an antibody for CK determination.

CONSTITUTION: This new gene has the base sequence given in the formula and codes H-chain V region of CKA-monoclonal antibody which inhibits the M subunit but not the M' subunit of CK. It can determine the serum creatinine kinase activity and is used for producing the CKA-5 monoclonal antibody useful for determination of the subunit which is effective for the early-stage diagnosis of myocardial infarction. This gene is obtained by extracting mRNA from the mouse hybridoma CKH-5 (FERM P-9839) producing CKA-5 monoclonal antibody and synthesizing cDNA using the mRNA and cloning the cDNA by the PCR process.