 - PIK3CA-related overgrowth spectrum PROS is caused by changes in the PIK3CA gene. This gene makes a protein that communicates with other proteins in the body to cause cells to grow. Alterations in PIK3CA change the chemical signals in the body and cause overgrowth in fatty, vascular and other tissues. Sirolimus is a drug that reduces the signals sent by one of the proteins in this chemical signaling pathway. Researchers want to learn whether the drug sirolimus can reduce or stabilize some of the overgrowth that patients with PROS experience. 