The Gal4 system quesyion

Hello,

in terms of the Gal4 system, my understanding is that its a transcription factor that drives the expression of a gene (any gene x) downstream, hence we can identify the gene x (gene of interest). Please correct me if im wrong

My question is, I am reading a paper about a gene that is involved in the circadian rhythem in fruit fly, and the author wanted to locate the loci of that gene by restricting the gene products to the Pigment-dispersing factor (pdf) at the ventral neurons by using Pdf-Gal4 . what do they mean by using Pdf-Gal4?


thanks