A possible mechanism for the role of Nej1p in NHEJ. In diploid cells, expression of the Mata1-Matα2 repressor (red circles) results in transcriptional repression of NEJ1. This, in turn, causes a loss of the nuclear localization of Lif1p (yellow), thus disabling the Lig4 complex. In contrast, haploid cells express Nej1p (green), which facilitates the transport of Lif1p into the nucleus where it forms the Lig4 complex, enabling the cell to carry out NHEJ. In the lower diagrams the proteins involved in NHEJ are in blue; see text for more details.