| Regulation of the cyclin D 1 and cyclin A 1 promoters by B Myb is mediated by Sp 1 binding sites . ^^^ Finally , we have found that the promoter of the cyclin A 1 gene is also activated by B Myb by a Sp 1 binding site dependent mechanism . ^^^ The effect of B Myb on the promoters of the cyclin A 1 and D 1 genes is reminiscent of the mechanism that has been proposed for the autoregulation of the B myb promoter by B Myb , which also involves Sp 1 binding sites . ^^^ |