The pET-21a-d(+) vectors carry an N-terminal T7•Tag® sequence plus an optional C-terminal His•Tag® sequence. These vectors differ from pET-24a-d(+) only by their selectable marker (ampicillin vs. kanamycin resistance). 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.