1) Eukaryotic genes can be organized in complex ways, including overlapping genes where coding sequences partially overlap, and split genes where coding sequences are interrupted by non-coding intron sequences. 2) Overlapping genes were discovered in bacteriophage X174, where the coding sequences of genes D and E overlap but are translated in different reading frames. 3) Split genes have exons, which are the coding sequences included in mRNA, and introns, which are intervening non-coding sequences not included in mRNA. Split genes were first observed in animal viruses in 1977.