Neurotrophins cluster into four ortholog groups which derive from two rounds of whole genome duplication (2R-WGD). Human neurotrophins (TF106463) include neurotrophin 4 (NTF4; located on chromosome 19), brain-derived neurotrophic factor (BDNF; chromosome 11), neurotrophin 3 (NTF3; chromosome 12) and nerve growth factor (NGF; chromosome 1). All other vertebrate neurotrophins can be clustered in clear ortholog groups with these four human genes. Phylogenetic timing places the duplications which gave rise to the four groups at the base of vertebrates. Red node dots signify duplication nodes, while green node dots signify speciation nodes. Labels "Vertebrata (2R)" signify nodes corresponding to 2R-WGD.