This study demonstrates the feasibility of generating thousands of transgenic Drosophila melanogaster lines where an exogenous gene is reproducibly expressed in distinct small subsets of cells in the adult brain. The researchers generated 5,200 DNA fragments from the flanking noncoding and intronic regions of genes thought to have patterned expression in the adult brain. These fragments were inserted into a defined genomic location using site-specific recombination and then tested for their ability to function as transcriptional enhancers in conjunction with a synthetic core promoter. Analysis of 44 fragments from four genes showed that over 80% drove expression patterns in the brain, with average patterns comprising fewer than 100 cells. The results suggest that the Drosophila genome contains over 50,000 enhancers, and that multiple enhancers drive distinct subsets of expression of a gene in each tissue and developmental stage. The study highlights the importance of enhancers in generating gene expression patterns and their potential as tools for neuroanatomy and neurogenetics. The researchers developed a strategy to generate transgenic lines that could be used to study the function of individual neurons. They used a synthetic core promoter that could work with a wide variety of enhancer types and tested the enhancer activity of DNA fragments from various genes. The results showed that the synthetic core promoter was effective in driving expression patterns and that the enhancer activity was highly reproducible across different animals. The study also found that the expression patterns driven by individual fragments were highly dynamic during development and that the patterns generated by the same enhancer in the adult brains of different animals were highly reproducible. The researchers concluded that it should be possible to establish a collection of transgenic Drosophila lines, each directing expression to a small subset of cells in the adult brain, and that in sum, would cover all cells in the brain. The study also highlights the potential of these lines for neuroanatomy and neurogenetics, as they provide a way to study the function of individual neurons and the organization of neuronal circuits in the fly brain. The results suggest that enhancers are reused throughout development and that they can drive expression in subsets of cells that do not normally express the endogenous gene. The study also highlights the potential of these lines for studying the function of individual neurons and the organization of neuronal circuits in the fly brain.This study demonstrates the feasibility of generating thousands of transgenic Drosophila melanogaster lines where an exogenous gene is reproducibly expressed in distinct small subsets of cells in the adult brain. The researchers generated 5,200 DNA fragments from the flanking noncoding and intronic regions of genes thought to have patterned expression in the adult brain. These fragments were inserted into a defined genomic location using site-specific recombination and then tested for their ability to function as transcriptional enhancers in conjunction with a synthetic core promoter. Analysis of 44 fragments from four genes showed that over 80% drove expression patterns in the brain, with average patterns comprising fewer than 100 cells. The results suggest that the Drosophila genome contains over 50,000 enhancers, and that multiple enhancers drive distinct subsets of expression of a gene in each tissue and developmental stage. The study highlights the importance of enhancers in generating gene expression patterns and their potential as tools for neuroanatomy and neurogenetics. The researchers developed a strategy to generate transgenic lines that could be used to study the function of individual neurons. They used a synthetic core promoter that could work with a wide variety of enhancer types and tested the enhancer activity of DNA fragments from various genes. The results showed that the synthetic core promoter was effective in driving expression patterns and that the enhancer activity was highly reproducible across different animals. The study also found that the expression patterns driven by individual fragments were highly dynamic during development and that the patterns generated by the same enhancer in the adult brains of different animals were highly reproducible. The researchers concluded that it should be possible to establish a collection of transgenic Drosophila lines, each directing expression to a small subset of cells in the adult brain, and that in sum, would cover all cells in the brain. The study also highlights the potential of these lines for neuroanatomy and neurogenetics, as they provide a way to study the function of individual neurons and the organization of neuronal circuits in the fly brain. The results suggest that enhancers are reused throughout development and that they can drive expression in subsets of cells that do not normally express the endogenous gene. The study also highlights the potential of these lines for studying the function of individual neurons and the organization of neuronal circuits in the fly brain.