The Spectra Backflow Protector Replacement is designed for the Spectra S1, S2, SG and 9 Plus Breast Pumps. It is a pivotal component of the Spectra closed system. The Spectra Backflow Protector ensures your breast milk is free of outside contaminants and prevents Breast Milk from making its way into the Breast Pump motor. The Spectra Backflow Protector is specifically designed to maintain the performance and efficiency of all Spectra Breast Pump models. If you are double pumping, you'll require two Spectra Backflow Protectors.