Exogenous addition of purified NF-YA subunit is able to reconstitute NF-Y binding activity in freshly isolated monocytes. Whole cell extracts obtained from human freshly isolated monocytes were incubated with the ³²P-labeled oligonucleotide corresponding to the MHC class II CCAAT box. Additions of 100 ng of the recombinant NF-YB subunit (lane 1) or 100 ng of the recombinant short form of NF-YA subunit (lane 2) were made to the reaction mix before EMSA where indicated. In lanes 3 and 4, 10 or 20 mmol/L DTT was added to the reaction mix. Whole cell extracts prepared from freshly isolated monocytes (d0) and fully differentiated macrophages (d7) (lanes 5 and 6) were used as control of NF-Y binding activity.