X-gal
A common abbreviation for 5-bromo-4-chloro-3-indolyl--D-galactoside. A sensitive, color indicator for -galactosidase. X-gal is a colorless compound but upon hydrolysis by beta-galactosidase releases an indolyl moiety. This product is not colored, but dimerizes after oxidation to form an insoluble and highly colored indigo dye. The oxidation is promoted by atmospheric oxygen, hence the color does not form anaerobically. (X-gal is also sometimes written as Xgal, without the hyphen.)

X-glu
A common abbreviation for 5-bromo-4-chloro-3-indolyl-glucuronide. A color indicator for beta-Glucuronidase (GUS). X-glu (AKA X-gluc) is a colorless compound but upon hydrolysis by GUS the indolyl moiety is oxidized to form a blue colored product. GUS is often used as a reporter gene in organisms that have high background levels of beta-galactosidase, often due to multiple genes encoding lacZ homologues. (See X-gal)

X-over
An abbreviation for cross-over.

X-P
A common abbreviation for 5-bromo-4-chloro-3-indolyl phosphate. A sensitive, color indicator for alkaline phosphatase activity. X-P is a colorless compound but upon hydrolysis by a phosphatase the indolyl moiety is released, and upon oxidization it forms a blue colored product. (See X-gal)