Enhancerless GFP reporter constructs used in research conducted in mice contain a generic mouse gene promoter (TATA box) followed by a transcription start site and then a GFP gene cDNA. When such enhancerless reporter gene constructs are integrated into random locations in the mouse genome, about 10% of the time, high levels of tissue-specific GFP expression are detected, even though the reporter gene construct itself has no enhancer. How is this possible

the reporter gene can randomly insert near to an enhancer sequence which can induce its expression

Explanation

Enhancers are genetic sequences capable of activating gene expression by binding to specific proteins (e.g., transcription factors). Enhancers can regulate the expression of nearby genes located thousands of nucleotides away, i.e., over several kilobases away. In the human genome, it is well known that enhancers are scattered across the 98% of the genome. In this case, it is expected that the reporter GFP gene construct is randomly inserted near an enhancer sequence (a 10% chance of insertion), thereby being regulated by that enhancer.