"P-199 is definitely the next step in skincare," says Somerville, who works with Lea Michele. "In the ‘90s we had glycolic acid. In 2000 it was all peptides. Stem cells are really where skincare is headed." Her CytoCell Dermal Energizing Treament ($150; katesomerville.com) contains the cutting-edge ingredient along with moisturizing properties like shea butter and soothing lavender.