RNA polymerase II requires the assistance of protein factors to bind to the core promoter. This requirement was first demonstrated by studying specific initiation at the major late promoter of adenovirus DNA, which controls highly expressed genes for structural proteins in the virus particle. RNA polymerase II cannot catalyze specific initiation at this promoter but gains the ability to do so when a soluble cell-free extract from human KB cells (cells derived from an oral epidermoid carcinoma) is added to it. In 1979, Robert G. Roeder and coworkers used classical protein fractionation techniques to isolate protein factors from the KB cell extract that assist RNA polymerase II. These protein factors, or general transcription factors (GTFs) as they are now known, were named TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH. The first two letters, TF, indicate the protein is a general transcription factor; the Roman numeral II signifies the factor supports RNA polymerase II transcription; and the final letter was assigned based on the protein fractionation scheme rather than on protein function. (The letters C and G are missing because later studies showed that the proteins originally assigned these letters afe not transcription factors for RNA polymerase II.) Subsequent studies by Roeder and other investigators demonstrated that these general transcription factors are present in all eukaryotes from yeast to humans. Moreover, archaea have similar general transcription factors. Counterparts in other eukaryotes serve the same functions.