The distribution of gene orders of 54 mitochondrial genomes from the Passeriformes based on phylogenetic tree. GO1, CYTB/tRNAThr/CR1/tRNAPro/ND6/tRNAGlu/CR2 (or NC)/ tRNAPhe; GO2, CYTB/tRNAThr/tRNAPro/ND6/tRNAGlu/CR/tRNAPhe; GO3, CYTB/tRNAThr/ tRNAPro/ND6/tRNAGlu/CR1/tRNAPro/ND6/tRNAGlu/CR2/tRNAPhe.