Gene duplication timing and spatial expression domain of progeny genes. The heatmap (false-color image) reveals the expression signature of vertebrate-specific gene duplications in the context of the broader evolutionary history of the animal kingdom. Tissues (horizontal axis) and taxons (vertical axis) were ordered using a simple hierarchical clustering algorithm (as visualized using dendrograms). The taxons are grouped into four clusters: (a) Eukaryota and Metazoa; (b) Bilateria, Chordata and Vertebrata; (c) Tetrapoda, Amniota, Mammalia, Eutheria and Theria; and (d) Catarrhini, Homo/Pan/Gorilla and Homo sapiens. Two round (2R) genes (labeled "Vertebrata") appear to be preferentially expressed in nervous tissues (amygdala, thalamus, caudate nucleus, corpus callosum, spinal cord, fetal brain, cerebellum, cortex and whole brain; highlighted in red), whole blood (in bold font), female reproductive track (uterus and ovary; highlighted in blue), several vertebrate-specific glands (salivary gland, prostate cancer, prostate, pancreas; highlighted in green) and the respiratory system (trachea and lung; in italics).