Ligases are enzymes that catalyze the joining of two molecules, or two groups within a single molecule, by forming a new chemical bond, with the concomitant hydrolysis of the diphosphate bond in ATP or a similar triphosphate.
Notes on Group
Despite their common/historical name, the UBIQUITIN LIGASES (FBgg0000069) are correctly classified as 'transferases' rather than 'ligases', and so do not appear within the LIGASES gene group.