what are 'floxed' genes in interbred mouse - (Apr/29/2008 )

I read a paper that interbred mouse with lung cancer (LSL-Kras) with mice bearing a conditional allele for Dicer 1 gene. Now the interbred mice were labelled as LSL-Kras that were wild type, heterozygous and homozygous for the conditional allele Dicer 1 gene (referred to as LSL KRAS Dicer +/+, LSL-Kras Dicer flox/+, and LSL Kras Dicer1 flox/flox, respectively). What does 'Flox' mean?

paper: impaired microNA processing enhances cellular transformation and tumorigenesis, nature genetics 2008 by Madhu Kumar.

Thanks
SF

this definition in wikipedia may help:

floxed