500px for iPhone saw a major upgrade to its iOS app in November.
But you won't find the app in Apple's U.S. App Store today.

The image sharing service 500px is reporting that Apple has pulled its mobile application from the U.S. App Store.

500px COO Evgeny Tchebotarev told TechCrunch that both the 500px for iOS and ISO500 for 500px were pulled from the Apple App Store over concerns about nude imagery available through the apps. The latter app appeared to be working at the time of this post, but the 500px app couldn't be found.

500px COO Evgeny Tchebotarev confirmed that users could turn off "safe search" on the platform by making a change via the desktop version of 500px, which effectively would allow users to search for photos with nudity on the app. However, the default "safe search" mode does hide these types of images, and Tchebotarev says the nude photos shared on the site are artistic in nature. Pornography is not allowed. 

500px is currently working on an update to its apps that will make nude images easier to identify and tag, and therefore more them more difficult to discover, but until then, the 500px app will apparently remain missing from the App Store.