Although the understanding of ectopic expressions deals with endogenous genes in an organism, it can be expended to a similar concept like transgenesis, which an exogenous gene is introduced to a cell or tissue type in which the gene is not usually expressed. Practices of ectopic expression in biological science is not only limited to identifying a function of the gene in a known cell or tissue type but also implemented to discover unknown or additional functions of the gene by ectopic expression.