Hello there Didik

If I understand correctly, you are performing transfection with a construct expressing the myc tagged BLV env gene or are you transfecting the gene fragment directly?

Could you specify which technique you are using to detect the myc tagged protein? Immunofluorescence imaging or WB?

Which cells are you transfecting in this experiment?

Some common reasons to not see tagged protein in exogenous expression systems can be

1. Lack of expression of that gene. If its a construct with a promoter not suited for the cells or if the gene fragment itself does not integrate into the genetic locus.

2. Rapid clearance of exogenous protein. Some cells are sensitive to foreign proteins and can degrade them through proteosomes super quickly.

3. Misfolding of target protein. This may cause your myc epitope to not be exposed, hence no signal (if you are using WB, denaturing the sample should help expose your protein)

4. Not sure about viral sequences so this is just a wildcard: was the gene codon optimized for your expression system? Sometimes pathogenic sequences are not expressed without other pathogen components in the host cell, so when expressing only one gene....it must be compatible with host cell for codon usage.

I hope this helps and good luck with your experiment

Best

Aparajita