The MRN complex is composed of Mre11, Rad50 and Nbn (known as Nbs1) proteins – for more information check PMID 21533982 and references within.
Mre11 is a 708 amino acid (human, 706 in rat and mouse) DNA –binding protein that also has single-strand endonuclease and double-strand 3’-5’ exonuclease activity.
Rad50 has two ATP-binding cassette (ABC) ATPase domains located at N- and C-termini that contain Walker A and B nucleotide-binding motifs. The 1,312 amino acid protein exhibits adenylate kinase activity and is important for tethering the complex to DNA.
The 754 amino acid (human, 750 in rat and 751 in mouse) Nbn protein contains forkhead-associated (FHA) and Brca1 C-terminal (BRCT) domains at the N-terminus and non-overlapping Mre11- and ATM-binding domains at the C-terminus.

The three genes are listed with aliases in parentheses, full names and links to gene report pages:
Mre11a (Mre11, meiotic recombination 11 homolog A (S. cerevisiae)) – MRE11 homolog A, double strand break repair nuclease
Rad50 (NBSLD, RAD50 homolog (S. cerevisiae)) – RAD50 double strand break repair protein
Nbn (Nbs, Nbs1, Nijmegen breakage syndrome 1, ATV) – nibrin