This reference is used to assign genes exhibiting a strong testis-biased expression to 'testis-specific' gene groups (protein complexes or pathways), based on high-throughput expression data in FlyBase and the 'testis tissue specificity index' given in FBrf0240104 (Additional file 2). Testis-specific gene groups may contain a mixture of ubiquitously expressed and testis-specific genes, for example for macromolecular complexes which have alternative paralogous subunits.