EMSA shows a protein complex containing PU.1 binds CD20 promoter site 2. (A) EMSA with either the PU.1 site of the SV-40 enhancer (PU.1) or the in vivo protected site 2 of the CD20 promoter (CD20#2) were incubated with BJA-B nuclear extract and the indicated unlabeled competitor probe (200-fold excess). The complexes detected with the CD20#2 probe are designated complex 2^A and 2^B. (B) EMSA with BJA-B nuclear extracts depleted of PU.1 or not using the CD20#2 probe. As in panel A, binding was competed with unlabeled CD20#2 probe (lane 2) or SV-40 PU.1 probe (lane 3). BJA-B nuclear extracts were incubated with a rabbit PU.1 antiserum (lane 4) or a control antiserum (lane 5) and cleared with goat antirabbit Ig coupled to magnetic beads before the EMSA.