Hi, I have the 13" retina MB Pro early 2015 and I purchased the Dell P2415Q which has a native resolution of 3840x2160. The 13" rMB Pro early 2015 is capable of 3840x2160 @ 60Hz through the mini display port. I have it connected to the mini DP port from the rMB to the DP of the monitor. The Dell packaging does not indicate whether the cable is v1.2. Anyhow, I am not able to select the default resolution of 3840x2160. The default resolution it is displaying is 1920 x1080. When I select 'About this Mac': it does report that it detects the DELL monitor, 24" (3840x2160) Intel Iris Graphics 6100 1536 MB graphics. The option resolutions are as follows: 1504x896 2304x12096 2560x1440 3008x1692 Could it be that perhaps that Dell supplied a v1.1 mDP to DP cable? Any advice on how I can resolve this would be appreciated. --- Post Merged, Feb 29, 2016 --- I figured it out. You have to select alt/option while clicking on 'scaled' in Display under System preferences to reveal the extra resolution options.