Three-letter abbreviations are used to describe phenotypes in bacteria including E. coli.
Examples include:
* Lac (the ability to use lactose),
* His (the ability to synthesize the amino acid histidine)
* Mot (swimming motility)
* Sm (resistance to the antibiotic streptomycin)
In the case of Lac, wild type cells are Lac and are able to use lactose as a carbon and energy source, while Lac mutant derivatives cannot use lactose.  The same three letters are typically used (lower-case, italicized) to label the genes involved in a particular phenotype, where each different gene is additionally distinguished by an extra letter.  The lac genes encoding enzymes are lacZ, lacY, and lacA.  The fourth lac gene is lacI, encoding the lactose repressor—"I" stands for inducibility.
One may distinguish between structural genes encoding enzymes, and regulatory genes encoding proteins that affect gene expression. Current usage expands the phenotypic nomenclature to apply to proteins:  thus, LacZ is the protein product of the lacZ gene, β-galactosidase.  Various short sequences that are not genes also affect gene expression, including the lac promoter, lac p, and the lac operator, lac o.  Although it is not strictly standard usage, mutations affecting lac o are referred to as lac o, for historical reasons.