Crucial 4GB Single DDR3 1866 MT/s (PC3-14900) 204-Pin SODIMM RAM Upgrade for iMac (Retina 5K, 27-inch, Late 2015) - CT4G3S186DJM Increases system performance Compatible with Apple iMac (Retina 5K, 27-inch, Late 2015) iMac17,1 only All memory stick modules extensively tested at the component and function level Meets or exceeds, Apple's quality standards and requirements Easy installation