The MRN protein complex, consisting of MRE1, RAD50 and NBS1, plays a crucial role in sensing DNA double-strand breaks (DSBs), and it is involved in cell cycle control. This makes the MRN complex an important guard of genome stability. Hypomorphic mutations in NBS1 result in the Nijmegen breakage syndrome (NBS), which is characterized by, among other things, an increased predisposition to malignancies, especially leukemia/lymphoma. Relatives of NBS patients carrying heterozygous mutations are also more prone to cancer development. This review summarizes several studies searching for associations between heterozygous mutations in NBS1, MRE11, and RAD50 and cancer and examining the levels of expression of proteins coded by these genes in tumor tissues. The results indicate that both decreased and increased expression of NBS1 may contribute to tumorigenesis, whereas overexpressed RAD50 has an anti-tumoric effect. MRE11 and RAD50 are also affected in tumors with microsatellite instability. However, the outcomes of association studies, which concerned primarily lymphomas/leukemias and breast cancer, were inconclusive. Heterozygous NBS1 mutations and molecular variants 657del5, I171V, R215W and E185Q were most commonly analyzed. Among these, an association with cancer was found most frequently for 657del5 (in leukemia/lymphoma and breast cancer) and I171V (in leukemia, breast, head and neck and colorectal cancers); however, other studies gave contradictory results. For other NBS1 as well as MRE11 and RAD50 variants, too little data were available to assess their role in cancer risk. Overall, the results suggest that heterozygous MRN complex mutations and molecular variants may contribute only to a limited fraction of tumors. This may be caused by several factors: various frequencies of the variants in specific populations, different criteria used for selection of control groups, possible effects of environmental factors, and potential interactions with variants of other low-risk genes. These issues, as well as the impact of the alterations on protein function, need to be addressed in future studies.