Polynucleotides, such as aptamers, comprising at least first one 5-position modified pyrimidine and at least one second 5-position modified pyrimidine are provided, wherein the first and second 5-position modified pyrimidines are different. Methods of selecting and using such polynucleotides, such as aptamers, are also provided.提供包含至少一種第一5位修飾嘧啶及至少一種第二5位修飾嘧啶之聚核苷酸,諸如適體,其中該等第一及第二5位修飾嘧啶不同。亦提供選擇及使用諸如適體之該等聚核苷酸之方法。