A DNA molecule having a gene expression repressing function derived from human T-cell leukemia virus type I (HTLV-I) existing in a region which is missing in a mutant provirus that is expressing p21Xm RNA but exists in the genome of a complete provirus, and a plasmid including the DNA molecule are provided. Furthermore, a novel protein (TRP-1) which specifically binds to U5RE and a structural gene for the protein is provided, which can be useful for elucidation of the transcription repression activity and elucidation of the oncogenesis mechanism of neurocytes, in which a transcriptional repressive region (U5RE) existing in the U5 region of human T-cell leukemia virus type I gene LTR is involved. Furthermore, an expression vector including the gene, a transformant into which the expression vector is introduced, and a process for producing the TRP-1 protein using the transformant are provided.