For example, word-type features form the backbone of most POS-tagging systems, but types like "gene" and "pathway" show up frequently in biomedical literature, and rarely in newswire text.
a classifier trained on newswire data and tested on biomedical data will have seen few training examples related to sentences with features "gene" and "pathway" (Ben-David et al., 2009;Blitzer et al., 2006).