The present invention relates to a pyrimidine compound or a pharmaceutically acceptable salt thereof represented by the following formula [I] ##STR00001## wherein each symbol is as defined in the specification and a method of therapeutically or prophylactically treating an undesirable cell proliferation, comprising administering such a compound. The compound of the present invention has superior activity in suppressing undesirable cell proliferation, particularly, an antitumor activity, and is useful as an antitumor agent for the prophylaxis or treatment of cancer, rheumatism, and the like. In addition, the compound of the present invention can be a more effective antitumor agent when used in combination with other antitumor agents such as an alkylating agent or metabolism antagonist.