Over the last five years, the number of clinical trials involving AAV 
(adeno-associated virus) and lentiviral vectors continue to increase by about 
150 trials each year. For continued success, AAV and lentiviral expression 
cassettes need to be designed to meet each disease's specific needs. This review 
discusses how viral vector expression cassettes can be engineered with elements 
to enhance target specificity and increase transgene expression. The key 
differences relating to target specificity between ubiquitous and 
tissue-specific promoters are discussed, as well as how endogenous miRNAs and 
their target sequences have been used to restrict transgene expression. 
Specifically, relevant studies indicating how cis-acting elements such as 
introns, WPRE, polyadenylation signals, and the CMV enhancer are highlighted to 
show their utility for enhancing transgene expression in gene therapy 
applications. All discussion bears in mind that expression cassettes have space 
constraints. In conclusion, this review can serve as a menu of vector genome 
design elements and their cost in terms of space to thoughtfully engineer viral 
vectors for gene therapy.