核酸
核糖核酸（RNA）由核糖核苷酸构成，其功能包括遗传信息的传递与核酶等，而一些病毒使用RNA携带遗传信息。组成RNA的碱基中，尿嘧啶（U）代替了胸腺嘧啶。RNA一般为单链。
长链非编码核糖核酸
对Alu元件的RNA转录物中的功能序列进行分析后，发现其亦有类似于蛋白质转录因子中结构域的模块化结构（Shamovsky 2008）。Alu元件RNA包括两个“臂”，每个臂都可以结合到一个RNAPⅡ分子上；体外实验表明该RNA还具有两个调控结构域，起到抑制RNAPⅡ转录活性的作用（Mariner 2008）。 These two loosely-structured domains may even be concatenated to other ncRNAs such as B1 elements to impart their repressive role (Mariner & Walters 2008). The abundance and distribution of Alu elements and similar repetitive elements throughout the mammalian genome may be partly due to these functional domains being co-opted into other long ncRNAs during evolution, with the presence of functional repeat sequence domains being a common characteristic of several known long ncRNAs including Kcnq1ot1, Xlsirt and Xist (Mattick 2003; Mohammad 2008; Wutz 2002; Zearfoss 2003).