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The size limit of AAV vectors is approximately 5 kb. Some serotypes may accept larger genes, but most literature cites success in packaging approximately 4.4 to 4.7 kb. Several publications have investigated the effects of exceeding the AAV payload (e.g., Grieger and Samulski in J Virol 2005, doi: 10.1128/JVI.79.15.9933-9944.2005 and Wu et al. in Mol Ther 2010, doi: 10.1038/mt.2009.255).
In general, approaching the limit will affect the packaging efficiency: the closer to the size limit, the lower the titer. However, there are many published reports describing alternative strategies for packaging larger transgenes, such as split AAV vectors and fragment assembly.