SAGA (Spt-Ada-Gen5 histone acetyltransferase complex) is a histoneacetylase complex which has Gcn5p as catalyst subunit and functions overlapping with the fundamental transcription factor TFIID which has Taf1p as catalyst subunit. However, SAGA and TFIID have different allotment and each accomplishes the important role in the transcription for Housekeeping gene group and Stress Responding gene group. Also, SAGA is a gigantic protein complex which is composed of Ada protein group (5 kinds), TBP related protein group (4 kinds), TAF protein group (5 kinds that also pertain to TFIID), and other protein groups (>6 kinds). As its molecular function, there is the chemical modification of histone, or the recruitment by direct interaction of transcription regulating factor on DNA, or the control of transcription starting reaction by TBP. Gcn5p that show histoneacetylase activity is one kind of the above-mentioned Ada protein group and in the case of budding yeast it is composed of 439 amino acid recidues (aa).