MUS81 and Fanconi anemia: In particular, this study demonstrated that the H. volcanii Hef helicase/endonuclease from the DNA repair endonuclease XPF (XPF)/Crossover junction endonuclease MUS81 (MUS81)/Fanconi anemia, complementation group M (FANCM) family of endonucleases (helicase-associated endonuclease fork-structure DNA) is essential for cell viability only in the absence of the Holliday Junction resolvase Hjc (8).