pET-31b质粒是一种原核表达载体，C端含有一个6×His标签，表达由宿主细胞提供的T7 RNA聚合酶诱导，目的基因被克隆到质粒载体上，受噬菌体强转录及翻译信号控制。
pET系统是有史以来在大肠杆菌中表达重组蛋白的功能最强大的系统，也是现今原核表达方面使用最广泛的系统。该系列质粒能很容易的通过降低诱导物的浓度来削弱蛋白表达。在非诱导条件下，可以使目的基因处于完全沉默状态而不转录。
The pET-31b(+) vector is designed for cloning and high-level expression of peptide sequences fused with the 125aa ketosteroid isomerase protein (1). The unique AlwN I cloning site allows the unidirectional insertion of of tandemly repeated peptide coding regions separated by methionine codons. Unique sites are shown on the circle map. Note that the sequence is numbered by the pBR322 convention, so the T7 expression region is reversed on the circular map. The cloning/expression region of the coding strand transcribed by T7 RNA polymerase is shown below. The f1 origin is oriented so that infection with helper phage will produce virions containing single-stranded DNA that corresponds to the coding strand. Therefore, single-stranded sequencing should be performed using the T7 terminator primer .
