DNA can be damaged by a number of things, such as ultraviolet light, radiation, oxidation agents, and alkylating agents. The primary function of DNA Ligase is to replicate, recombine, and repair DNA. There are two classes of DNA Ligase: NAD-Dependent and ATP-Dependent. Only ATP-Dependent forms of the enzyme are found in mammalian cells. In humans, there are four distinct forms: DNA Ligases I, II, III, and IV.
    While all DNA ligases are involved in DNA repair mechanisms, each of the four have distinct functions in various pathways. Their structures differ slightly, leading to the binding to different proteins and repair sites for each DNA ligase form. DNA ligases I and III are the most abundant of the four types found in the mammalian body. It has recently been suggested that DNA Ligase III, not DNA Ligase I, accounts for over 85% of the joining of DNA strands in mammalian cells. 

  Current research has led to following functions of each of the common types of DNA ligases found in humans:

• DNA ligase I: This form of the enzyme joins Okazaki fragments during lagging strand DNA replication and some recombinant fragments. It interacts with PCNA to aid in its functions. It is also involved in base-excision repair and nucleotide-excision repair

• DNA ligase II: This form is very similar to and is thought to be derived from DNA Ligase III. However, this form of ligase is found in non-dividing cells of mammals. (alternatively spliced form)
  

• DNA Ligase III:
  

                     DNA Ligase III includes a zinc finger, which aids in locating strand breaks needing repair. The two forms DNA
                     ligase-alpha and DNA ligase-beta differ in their c-terminal amino acid sequence. DNA Ligase
                     III-alpha found in all human somatic cells interacts with XRCC1, another DNA repair protein. DNA
                     Ligase III-beta, however, is only found in the testis and does not form a complex with XRCC1,  
                     suggesting that its serves functions other than DNA repair. DNA Ligase III is involved in meiosis. It  
                     has been suggested that this enzyme aids in the homologous recombinations that occur before the
                     first meiotic division.

• DNA Ligase IV:  It catalyzes the final step in double-strand break repair, while interacting with the XRCC4, a double-stranded break repair protein. It is also required for the process which generates diversity in cell loci during immune system development.  It has been suggested that DNA ligase IV is involved in early embryonic development. This was discovered when mice embryos were killed when the function of this enzyme.