Currently in the hobo system of hybrid dysgenesis, strain classification is based on the presence/absence of the 2.6 kb Xho I restriction fragment. Using this criterion, strains are classified as: (1) H strains when full-size elements are detected by presence of a 2.6 kb Xho I restriction fragment; they can also contain internally deleted elements; (2) DH strains when only deleted elements are detected (Xho I restriction fragment less than 2.6 kb); (3) E strains, devoid of any restriction fragment equal to or less than 2.6 kb in length. In addition, the strains can be classified on their ability to generate gonadal atrophy (GD sterility) when males of a studied strain are crossed with females from an E strain (dysgenic cross). Here we try to define the nature of the dysgenic cross, which leads us to analyse the different components of the dysgenic syndrome and to look for eventual correlations between them. Molecular analysis, GD sterility tests, hobo mobilization with the haw strain and the vg(al) strain, and hereditary transmission of the instability at the vg locus have been assayed in different strains. We show that the occurrence of GD sterility depends on the tested H strains as expected, but also on the E strains used. On the other hand we do not find any correlation between the different dysgenic parameters. Our data reveal that molecular and GD sterility tests are not sufficient to classify strains in the hobo system, and that all the components of the dysgenic syndrome must be taken into account. Our results are discussed with regard to active and full-size elements in relation to the structure of the S region where an amino acid sequence (TPE) presents a repetition polymorphism.