A tumor marker nucleic acid of the present invention is concerned with a nucleic acid hybridizing under stringent conditions to a nucleotide sequence described in SEQ ID NO: 1 or a complementary nucleotide sequence thereof. A method of testing canceration of the present invention is a method comprising diagnosing a biological sample as being cancerous when the transcription level of the nucleic acid in the biological sample significantly exceeds that in a normal biological sample as a control. The present invention also relates to a .beta.1,3-N-acetyl-D-glucosaminyltransferase protein having an activity of transferring N-acetyl-D-glucosamine from a donor substrate to an acceptor substrate through .beta.1,3-linkage.