Artificial microRNA–truncated CD34 (amiR–tCD34) vector design. Shown in the top diagram is the design principle for amiRs, which contain the 5′, loop, and 3′ sequences from a primary microRNA (miR) transcript with the stem sequence being derived from a small interfering RNA (siRNA). The MSGV2–tCD34 vector is shown in the bottom diagram with the location of the insertion sites for the amiR sequences. LTR, long terminal repeat; SD, splice donor; ψ+, extended packaging sequence; SA, splice acceptor.