If you look at the “Tree for I1” file at http://knordtvedt.home.bresnan.net (1 December 2011 version) you will see that I1 tree branch having Z131+ has not been found yet in FTDNA tests. But incorporating the 1000 Genomes view that Z131+ haplotypes exist, and probably fit into the tree upstream of a yet to be cataloged DF29 snp, one concludes any Z131+ haplotypes will be real outliers with lots of differences from the bulk of I1 haplotypes. If you have a extreme outlier I1 haplotype, you might consider testing for Z131, and probably Z58 and Z63 just to be safe you find a niche.