How to make X-a-Gal Indicator Plates

To make X-a-Gal indicator plates, X-a-Gal can be either spread on top of the medium or included in the medium prior to pouring the plates.
 
To spread X-a-Gal onto pre-made plates:
If using freshly poured plates, allow yeast dropout medium to cool down  and harden at room temperature. Spread 200-500 ml of X-a-Gal stock  solution (2 mg/ml in DMF) onto a 15-cm plate or 100-200 ml onto a 10-cm  plate evenly. Leave plates to dry at room temperature. Plate cells and  incubate at 30°C until blue colonies form.
 
To pour X-a-Gal indicator plates:
Autoclave the appropriate dropout agar medium. Let it cool to 55°C before  adding 1 ml of X-a-Gal stock solution (20 mg/ml in DMF) per 1 litre  medium. Pour plates and allow medium to harden at room temperature.  Plate cells and incubate at 30°C until blue colonies form.
 
Stock solutions of X-a-Gal should be stable for at least 6 months if kept in the dark at -20°C.