Percent (%) usually means mass/volumeFor example,if you want to make a gel with an agarose concentration of 0.5%,you would use 0.5 g agarose per 100 ml of solution. g/l,or μg/μl,or similar units (mass over volume). We’ll use these units for DNA and proteins,which are variable in size. X. You''ll use the mysterious X frequently in lab.