The RUNX1 response element in the promoter of the BLK gene, encoding B cell tyrosine kinase BLK, is adjacent to the ETS response element as well as the PAX5 (BSAP) response element. The RUNX1:CBFB complex can bind to the ETS family transcription factors ELF1 or ELF2, and this complex can bind to RUNX1 and ETS response elements in the BLK promoter (Cho et al. 2004). RUNX1 can also form a complex with PAX5 (BSAP), and this complex can bind to RUNX1 and PAX5 response elements in the BLK promoter (Libermann et al. 1999). It is not known whether a larger complex, consisting of ELF1 or ELF2, the RUNX1:CBFB complex and PAX5 is formed and if ELF1/ELF2, RUNX1 and PAX5 can simultaneously reside at the BLK promoter. PAX5 is known to contain and ETS binding region and it is possible that it can, through this region, interact with ELF1 and ELF2 (Cho et al. 2004).