Disclosed is an identification marker which can be utilized for detection of various human cancer cells and whose expression closely relates to malignant alteration of cells, and compositions for human cancer treatment which are based on suppression of cancer cell proliferation through inhibition of expression of the identification marker. The marker is human heterochromatin protein 1γ (HP1γ), and the compositions for cancer treatment comprises one or more agents which suppresses the expression of human HP1γ gene, such as siRNAs to human HP1γ.