G418 sulfate (also called Geneticin) is a antibiotic which inhibits translation by binding irreversibly to the ribosome. It is used in yeast to select for the presense of the KanMX marker. KanMX encodes an enzyme which renders the G418 molecule inactive.
We order G418 from Mediatech (cat #: 61-234-RG) and it's stored in the 4 degree fridge by Mark's bench in the yellow tube rack at the bottom of the fridge. A working stock solution in deionized water of 100 mg/ml is used in pouring selective plates which have a final concentration of 200 mg/L. Although recently it was found that increasing the concentration to 400 mg/L works better for YPD plates. Add the G418 solution to the plate media once it has cooled to about 50 Celsius (about 5 seconds before you have to remove your hand). G418 plates must be stored at 4 degrees Celsius.
To see how you make a G418 Plate follow this Link.