Three UniRef tables UniRef100 , UniRef90 and UniRef50 ) are available for download : UniRef100 combines identical sequences and sub-fragments into a single UniRef entry ; and UniRef90 and UniRef50 are built by clustering UniRef100 sequences into clusters based on the CD-HIT algorithm [ 16 ] such that each cluster is composed of sequences that have at least 90 % or 50 % sequence similarity , respectively , to the representative sequence .