Abstract: Biological materials are applied to a CHO cell or the like for producing a species of protein. The biological materials includes a first vector and a second vector, the first vector including a dhfr gene of a species of mammal and a gene of the species of protein, the second vector including a DNA fragment for inducing a RNA interference in the CHO cell to reduce expression of a dhfr gene of relevance in said CHO cell after the second vector is applied to the CHO cell. The dhfr gene of the species of mammal, or a dhfr gene of the CHO cell, or both constitute at least part of the dhfr gene of relevance after the first vector is applied to said CHO cell. The DNA fragment consists of nucleotides characterizing a segment of a dhfr gene of the CHO cell and a segment of a dhfr gene of the species of mammal.