A unique nested gene arrangement for the elav and arginase genes in D. melanogaster. A: The elav gene is nested inside the third intron of the arginase gene. Complementary strands are transcribed to generate the elav and arg RNAs with inverse polarities . B: Examination of the relative arg-elav arrangement in 11 metazoans. There are two arginase genes in humans, only one in the other examined species. Column 1 documents the status of the arginase third intron. Column 2 specifies the nested (+) or independent (-) arrangement of the arginase/elav genes. N.A.: Not applicable. The third column indicates the percentage of amino-acid sequence identity of D. melanogaster compared with other species. *: N-terminally truncated arginase sequence for P. humanus corporis. See Additional file 3 for arginase alignments.