Background: XRCC4-like factor (XLF) functions in DNA nonhomologous end joining (NHEJ) in double-strand break repair and immune system V(D)J recombination. Mutations of XLF in humans have been documented to result in immunodeficiency, growth retardation, and microcephaly. The yeast of homolog of XLF is Nej1. XLF has been shown to interact with the XRCC4 x DNA-ligase IV complex and is implicated in the DNA religation step of the NHEJ pathway.