PU.1 and Pip bind cooperatively to the CD20 promoter and the λB element. A total of 0.5 ng of the indicated labeled probe were incubated either with BJA-B nuclear extract (lanes 3, 4, 9, and 10), in vitro translated PU.1 (lanes 1 and 2), a combination of in vitro translated PU.1 and either in vitro translated Pip (lanes 5, 6, 8, and 12), unprogrammed reticulocyte lysate (lanes 7 and 11), or a tonsil B nuclear extract (lanes 13 to 16) in the presence of a preimmune antisera (lane 15) or anti-Pip antiserum (lane 16). The PU.1/Pip complex migrates slightly faster than a nonspecific complex that can be partially competed by any DNA probe. A 200-fold excess of respective unlabeled competitor probe was added in the indicated lanes (+). EMSA was performed as described in Fig 3. Lanes 1 to 8, lanes 9 to 12, and lanes 13 to 16 are from three separate experiments so the migration of PU.1 and PU.1/Pip are slightly different.