Gene stacking is the combination of more than one gene for plant disease resistance, or crop productivity, or other horticultural traits.^{[T 1]} In plant breeding traditionally that means breeding those genes in, but increasingly also can mean genetic engineering.^{[T 1]} This can be achieved a few different ways, and gene pyramiding is one of those methods.^{[T 2]} Stacking of transgenes is yet more difficult than stacking natural genes, but especially in the case of pest resistance genes which require a significant financial investment to insert, is advantageous over other methods.^[1] Pathosystems with rapid evolution in the pathogen have long been considered good targets of stacking, to broaden and prolong resistance.^[2][3]

Assaying for successful insertion of R genes is much more difficult than for one at a time.^[4] A simple challenge assay will only tell between complete failure/some unknown degree of success.^[4] Building a new technique specifically for the multiple genes you are attempting to insert may be necessary.^[4]