Regulatory genes important for development and physiology are often transcribed in tissue-and temporaryspecific manners. Differential gene expression is therefore an important aspect of genome function in multicellular organisms. Transcriptional enhancers are often difficult to identify by a simple survey of genomic sequences since their nucleotide sequence are not well conserved and may be located several tens of kilobases from the site of transcription initiation. Enhancer trapping detects chromosomal enhancer elements in situ with minimal perturbations of the genome structure (Bellen et al., 1989; Bier et al., 1989; O’Kane and Gehring, 1987). A database of enhancer trap elements compiling their expression patterns and precise genomic locations should be extremely useful for studies of gene expression and genome analyses. A previous study analyzed 1,045 lethal insertions of a P-lacZ enhancer trap element (Spradling et al., 1999). We report here a set of data obtained from analyses of 4,615 independent insertions of the P {GawB} enhancer trap element (Brand and Perrimon, 1993).

The screen was designed to collect balanced stocks of new P {GawB} insertions into all major chromosomes with as little bias as possible. New insertions were generated using the stocks and methods described in Yoshihara and Ito (2000) in the laboratories of SH, KI, FM, RU, TA, TT, TU, and MY In brief, the Gal4 enhancer trap element P {GawB}(Brand and Perrimon, 1993) carried on the CyO chromosome was mobilized in the male germline and insertions into other chromosomes were established and retained. Second-and thirdchromosome insertions were each balanced over