EE-Flipper 33 is a Flipper probe designed to evaluate the mechanism of early endocytosis. EE-Flipper has a pKa value of 10.6 and is capable of tracking approximately 70% of early endosomes (EE) in live cells. Furthermore, EE-Flipper 33 can also label the Golgi apparatus in live cells with an internal pH ranging from 6.0 to 6.7.