The gene encoding the Nin one binding (NOB1) protein which plays an essential role in protein degradationhas been investigated for possible tumor promoting functions. The present study was focused on NOB1 as apossible therapeutic target for breast cancer treatment. Lentivirus mediated NOB1 siRNA transfection wasused to silence the NOB1 gene in two established breast cancer cell lines, MCF-7 and MDA-MB-231, successfultransfection being confirmed by fluorescence imaging. NOB1 deletion caused significant decline in cell proliferationwas observed in both cell lines as investigated by MTT assay. Furthermore the number and size of the coloniesformed were also significantly reduced in the absence of NOB1. Moreover NOB1 gene knockdown arrested thecell cycle and inhibited cell cycle related protein expression. Collectively these results indicate that NOB1 playsan essential role in breast cancer cell proliferation and its gene expression could be a therapeutic target.