sentences:  It is known as a small nucleolar RNA (snoRNA) and also often referred to as a guide RNA.Z18 snoRNA belongs to the C/D box class of snoRNAs which contain the conserved sequence motifs known as the C box (UGAUGA) and the D box (CUGA).Most of the members of the box C/D family function in directing site-specific 2'-O-methylation of substrate RNAs.