16 Pa. Stat. § 1421
In all cases where more than one assistant district attorney is appointed, the district attorney shall designate one assistant as the first assistant. The first assistant or the assistant district attorney where only one is appointed shall, in the absence of the district attorney from the jurisdiction or during the district attorney's disability to perform the duties of the office through sickness or other cause, be vested with all the duties, powers and privileges given by law to the district attorney, and generally, at such time, be empowered to do and perform all things in connection with the office which the district attorney may by law be entitled to do or perform. In case of any incapacity of the district attorney or the first assistant, or both, any or all of such duties, powers and privileges may be done by such other assistant district attorneys, if any, as may be designated by the district attorney.
16 P.S. § 1421