The most common way to create fluorescent lines in zebrafish is to use the Tol2 transposon-based system, which contains the gene of interest associated with a fluorescent protein, such as eGFP. After injection of the construct, different embryos will have integrated a different number of repetitions, which results in different levels of fluorescence, so the line will need to be outcrossed with WT for several generations, to obtain individuals with only one repetition and stable fluorescence.