A method for detecting nonsense mutations and frameshift mutations in a test nucleic acid fragment which is simple and may be used even when the size of the test nucleic acid fragment is large is disclosed. The method involves inserting the test nucleic acid fragment into a vector containing a promoter, a translational initiation codon, and a reporter gene, and transforming a host cell with the vector to express a fusion polypeptide. The production of the fusion polypeptide is assayed for in the host cell, and non-production of the fusion polypeptide is indicative of a nonsense and/or frameshift mutation in the test nucleic acid fragment.