Check for toxicity: no protein may mean the cells have lost the expression plasmid or elements of the expression plasmid have been deleted. Culture cells for protein expression. Just before induction, plate a sample on duplicate plates with and without antibiotic selection. If toxicity is an issue, there will be a significant difference between the number of colonies on the plates. Fewer colonies will be seen on plates containing antibiotic (indicating that the plasmid has been lost) compared to plates without antibiotic. Check clone integrity by restriction enzyme analysis and/or sequencing of the ORF.