Retrieve human transcription factors from HumanTFDB, extracted from
AnimalTFDB 3.0: a comprehensive resource for annotation and prediction
of animal transcription factors. Hui Hu, Ya-Ru Miao, Long-Hao Jia,
Qing-Yang Yu, Qiong Zhang and An-Yuan Guo. *Nucl. Acids Res*. (2018).