Mass. Gen. Laws ch. 18 § 8
The provisions of section nine A of chapter thirty and the provisions of chapter thirty-one shall not apply to the office of commissioner, deputy commissioner or assistant commissioner. If an employee of the commonwealth or of a political subdivision, as defined in section one of chapter thirty-two, shall be appointed to the office of commissioner, deputy commissioner or assistant commissioner, he shall upon termination of his service in such office be restored to the position which he held immediately prior to such appointment. Such restoration shall be made without impairment of his civil service status or tenure under section nine A of chapter thirty and without loss of seniority, retirement or other rights to which uninterrupted service in such position would have entitled him. During the period of such appointment each person so appointed from a position in the classified civil service shall be eligible to take any competitive promotional examination to which he would have otherwise been eligible.
Mass. Gen. Laws ch. 18, § 8