Before revision:  Our results show that extrinsic factors quantitatively and qualitatively affect the histogram. Specifically, we{\it   suggest that the cell to cell variation in extrinsic factors accounts for the wider that Poisson{\it  distribution of mRNA copy numbers.
After revision:  Our results show that extrinsic factors qualitatively and quantitatively affect the probability distribution of the gene product. We work out, in detail, the transcription of mRNA from a constitutively expressed promoter in{\it suggest that the cell to cell variation in extrinsic factors accounts for the wider that Poisson{\it distribution of mRNA copy numbers.