Hygromycin B: Hygromycin B, an aminoglycoside antibiotic, inhibits the growth of prokaryotic and eukaryotic microorganisms and mammalian cells. It inhibits protein synthesis at the translocation step on the 70S ribosome and induces misreading of mRNA template.1-3 Hygromycin B has been used to select mutants in a wide variety of cells including bacteria.4,5 protozoans,6 yeast,7 fungi,8-12 plants,1,13-15 and mammalian cells.16-18 Resistance to Hygromycin B is determined by a gene coding for a phosphotransferase that phosphorylates Hygromycin B, thereby making it inactive.4,19 Hygromycin B is known to selectively penetrate cells that have been rendered permeable by virus infection.20 This, combined with its potency in inhibiting translation, makes it an effective antiviral agent.21-23 The effective concentration required for selection varies depending on the type of cells, media, growth conditions, and the stage of the cell cycle. The effective dosage also depends on cell density. At higher cell densities somewhat greater doses are required to kill non-resistant cells. Hygromycin B can kill most bacterial cells under aerobic conditions at concentrations ranging from 10 to 1000 μg/ml. The sensitivity to Hygromycin B can be increased by increasing the pH of the medium. Also, the sensitivity appears to be greater at lower salt concentrations