The Protomycetaceae are closely related to the better known Taphrinaceae. The limit between the two is vague: while the genera Protomycopsis, Taphridium, and Volkartia are placed by Alexopoulos a.o. in the Protomycetaceae, they are attributed to the Taphrinaceae by the Index Fungorum (2014). The main difference between the two families is that in the Taphrinaceae the asci are formed at the end of the hyphae, while in the Protomycetaceae they can originate at any point out of ascogenous cells, not necessarily therefore on the epidermis of the host plant, but also deeper in the tissue.
Alexopoulos, Mims & Blackwell (1996a).