Supplemental Instructions (SIs) were introduced into the San Francisco State University College of Science & Engineering curriculum in 1999. The goal was to improve student performance and retention and to decrease the time to degree in STEM majors. While for the most part we followed the structure and activities as developed by the International Center for Supplemental Instruction at the University of Missouri, Kansas City, we discovered several variations that significantly improved our outcomes. First and foremost, we created SI courses that require attendance, which results in higher students’ performance outcomes compared to drop-in options. Second, at SFSU the SI courses are led by pairs of undergraduate student facilitators (who are all STEM majors) trained in active learning strategies. Each year, more than half of our facilitators return to teach for another year. Thus, each section has a returning “experienced” facilitator who works with a new “novice” facilitator. Third, the SI courses were created with a distinct course prefix and listed as courses that generate revenue and make data access available for comparison studies. Results are presented that compare SI impact by gender and with groups underrepresented in STEM disciplines.