General transcription factor that plays a role in transcription initiation by RNA polymerase II (Pol II) (By similarity). Involved in the pre-initiation complex (PIC) formation and Pol II recruitment at promoter DNA (By similarity). Essential for early embryonic development (PubMed:11566890). Associates with TFIID-TFIIA (DA complex) to form TFIID-IIA-IIB (DAB-complex) which is then recognized by RNA polymerase II. RNAi-mediated knockdown causes arrested development at 90-100 cells and inhibits differentiation (PubMed:11566890). The two E cell daughters (E2 cells), which form the endoderm, divide abnormally early, immediately after the MS2 cells (PubMed:11566890). Phosphorylation of the RNA Pol II large subunit C-terminal domain (CTD) is reduced severely in embryos (PubMed:11566890). Belongs to the TFIIB family.