The mini-w mini-gene in many currently used Drosophila transformation vectors originates from the pW6 (Klemenz et al., 1987) and pCaSpeR (Pirrotta, 1988) P-element vectors. The mini-w cassette from pCaSpeR (w+mC) consists of ~300 bp of upstream and ~600 bp of downstream regulatory w sequence, with most of the first intron removed. In the pW6 vector, the minimal w promoter is replaced with the Hsp70 minimal promoter (w+mW.hs). Transformation of w null mutants with these vectors, or their many derivatives, rescues eye pigmentation through expression of White protein (i.e. causes eyes to be red). The convenient nature of the w eye color phenotype has made mini-w a routinely used marker of transgenesis in Drosophila. For example, most Gal4 and UAS transgenes used in flies are marked with mini-w (e.g. (Brand and Perrimon, 1993; Dietzl et al., 2007) and (at the time of manuscript preparation) more than 40% of the ~30,000 P-element vector-containing strains in the Bloomington Drosophila Stock Center (http://flystocks.bio.indiana.edu/) were marked with a version of mini-w. Although other markers for transgenesis are used in Drosophila (e.g. yellow and GFP (Bellen et al., 2011)), the mini-w marker has been used in many—bordering on all—genetic studies in flies.
