Possible model of human TFIID assembly. A number of TFIID subcomplexes initially assemble in the cytoplasm (5TAF: TAF4, 5, 6, 9, 12; cTAF: TAF2, 8, 10; sTAF: TAF1, 7, 11, 13 and TBP; TAF3/10) and can enter the nucleus due to the presence of nuclear localization signal sequences on TAF4, TAF8, TAF1 and TAF3. The final stages of TFIID assembly likely occur in the nucleus. We propose that the binding of either cTAF or TAF3/10 to the 5TAF leads to two different core subcomplexes, bcTAF and aTAF, respectively. Once the bcTAF and aTAF subcomplexes heterodimerize, they can then interact with the sTAF subcomplex to complete the assembly of TFIID.