Limited amount (5 - 10%) of spontaneous differentiation is not an issue in culture, as long as the areas of differentiation are removed on the day of passaging. hPSCs are prone to differentiation when grown in vitro. This is a characteristic of hPSCs: they have the ability to differentiate into cells of the three different germ layers. The pluripotency factors in TeSR media will help prevent the cells from differentiating, but a low level (< 10%) of spontaneous differentiation in culture is normal and healthy. Many labs use different techniques (e.g. expression markers, colony morphology, karyotyping, functional pluripotency tests, differentiation to three germ layers) to ensure their hPSCs are maintained in an undifferentiated state.