The Bloomington Stock Center has many stocks containing an X-linked insertion of a construct expressing FLP recombinase upon heat shock. To our knowledge, all of these insertions behave as one would expect and catalyze recombination between FRT sites. Nevertheless, there is some doubt regarding the identities of constructs and insertions in some stocks. This will not matter to most stock users, but the following presents our best understanding of this confusing situation.

Three constructs express FLP under control of an Hsp70 promoter. P{hsFLP} was described in Golic and Lindquist (1989) and P{hsp70-FLP} was described in Struhl and Basler (1993). The two constructs seem similar in most respects. Golic and colleagues later replaced sequences flanking the 3' end of FLP in P{hsFLP} to create P{70FLP} with higher FLP recombinase expression.

An X-linked insertion of P{hsFLP} called P{hsFLP}1 was isolated and is still in common use. Stocks 6 and 7 were the first P{hsFLP}1 stocks at Bloomington. Chou and Perrimon (1992) mobilized P{hsFLP}1 to recover the autosomal insertions P{hsFLP}38 and P{hsFLP}86E. Later, Chou and Perrimon (1996) mobilized P{hsFLP}38 to isolate the new X-linked insertions P{hsFLP}12 and P{hsFLP}22 with higher FLP recombinase activity than P{hsFLP}1. Stocks of all these insertions were sent to Bloomington. The P{hsFLP}22 stock (#1970) was lost in 2002. Stock 1929 is the original P{hsFLP}12 stock. An X-linked insertion of P{hsp70-FLP} called P{hsp70-FLP}1 was isolated. We list it as a component of stocks that can be traced to the Basler lab.

We are not 100% certain which X insertion is present in many stocks. Many stocks arrived with a generic "hsFLP" genotype. We have listed most of these insertions as P{hsFLP}12, because it appears to be the most widely used insertion. Other stocks arrived with the insertion identified as "122". We suspect that "122" is P{hsFLP}12 in most stocks and have listed it that way in genotypes.

David Bilder sequenced two hs-FLP stocks: P{hsFLP}1 from stock 26902 and a lab stock labeled "P{hsFLP}122", most likely derived from stock 1929. He reported that the transgenes in both stocks carry the D5 FLP variant (see the defined FLP page for more information on FLP variants). This suggests that P{hsFLP}38, P{hsFLP}86E, P{hsFLP}12, and P{hsFLP}22 also carry the D5 variant. Given the uncertainties surrounding the identities of most hsFLP transgenes on the X, we list only those for which we can be reasonably certain of their origin on the defined FLP page.

We recommend that you characterize your particular insertions molecularly if your goal is to make detailed comparisons. We note that for many experiments, maximal FLP recombinase activity may not be necessary. For example, hundreds of FRT-derived deletions and duplications have been isolated here using P{hsFLP}1, which is reported to be the least efficient insertion.

No similar ambiguity surrounds P{70FLP} insertions or autosomal P{hsFLP} insertions in Bloomington stocks.