I try to clone a gene, simply cut and paste it into another plasmid. I took the gene out with restriction enzymes blunt from one size and sticky from another site and inserted into blunt/sticky ends in the MCS of another expression vector. I've got positive clones, The DNA is ok, I can cut out the insert, I also sequenced the insert and it was ok. However when I transfect it into mammalian cell line, I don't see the expression. What could be a problem?