sentence: The C / D box snoRNAs , which consist of two major boxes , namely , C ( RUGAUGA , R = purine ) and D ( CUGA ) , function as a guide for position-specific 2 ′ - O-methylation of target molecules and are associated with four core proteins , methyltransferase fibrillarin ( FBL ) , NOP56 / NOL5A , NOP58 / NOP5 , and SNU13 / NHP2L1 , which constitute the core of C / D box snoRNPs [ 23 – 26 ] .