RRM2B and Huntington disease: The first GWAS of 4,082 HD patients using the difference between AO predicted by CAG length and actual AO of motor symptoms identified FAN1 (FANCD2 and FANCI associated nuclease 1) on chromosome 15 and RRM2B (ribonucleotide reductase regulatory TP53 inducible subunit M2B) on chromosome 8 [21].