The present invention relates to methods for determining the sequence bias of a sequencing technique. Furthermore, the invention relates to methods to reduce or enhance sequence bias during sequencing of nucleic acids via techniques involving adaptor ligations. Specifically the method relates to use of a degenerate RNA sequence to analyze sequence bias when generating small RNA libraries, and to the use of modified adaptors for cloning of small RNAs with degenerate or specific sequences to reduce or enhance sequencing bias, as well as various nucleic acid molecules relating thereto or derived therefrom.