(A) A cellular gene producing an mRNA endowed with a regulatory miRNA target sequence at its 3′ end. Protein product is described at right. (B) The same gene, with a vector provirus integrated between two exons in the sense orientation. Two general categories of aberrant mRNAs are depicted as either 5′ (av) or 3′ (va/vb) fusions between vector (v) and cellular transcripts. Compared with its physiological counterpart (a), av mRNA yields a truncated cellular protein (potentially fused to a fragment of the transgenic protein) at high levels, owing to the loss of 3′ miRNA target sequences. va results from proviral transcriptional read-through, and vb results from the use of a cryptic splice donor in the vector. Only the transgenic protein is produced at significant levels from va, as translation of the cellular part of this transcript would require reinitiation, a very inefficient process. The resulting transcript is predicted to be expressed at low levels, irrespective of the presence of an miRNA target sequence, due to nonsense-mediated degradation. (C) The provirus-harboring locus, with insertion of target sequences for a stage-specific miRNA in the vector transcript as a safeguard. Both vector-derived (v*) and cellular-viral fusion (av*, v*a) mRNAs will be degraded in cells expressing the miRNA, e.g., transformation-prone stem cells, resulting in very low levels of abnormal protein. However, a vb-like mRNA devoid of miRNA target sequence owing to aberrant splicing would escape downregulation, as would av-like transcripts generated from a provirus integrated in the antisense orientation.