Arabidopsis gene families are defined with all-against-all similality searches conducted using BLAST with an E-value cutoff of 1e-5 (Altschul et al., 1997). Based on the transformed E-values (Shiu et al., 2005), we generated similarity clusters representing gene families with the Markov clustering program (http://micans.org/mcl/ [van Dongen, 2000]). RIKEN PSC A.thaliana gene family data