1) Data were Chi-square tested under the null-hypothesis that L91' and E82' do not trans-silence their target loci, i.e., an additive interaction between the partner loci. Thus, in the case of L91 × E82, the only genotype escaping cosuppression should be L91 /-; E82 (-/-), which occurs at a frequency of 2/15 (13%). Note that entirely non-transgenic plants are not considered because of counterselection. Imprinted loci are not expected to cause endogene silencing (see Fig. 5). Thus, 12/15 of L91'/E82 family members, 4/15 of E82'/L91 family members and none of L91'/E82' family members are expected to be cop1-like. An excess of wild-type plants is inconsistent with an additive interaction and thus suggests trans-silencing activity by the L91' or L91 locus.