Table 1. Comparative table of several alignment-free approaches.

No. proteins	Information	Parameters	Classification method			Level	Work
1,371	3D structure	3D-HINT potential	LDA	QSAR	ANN	0–1	Concu et al. (2009c)
4,755		Moments, entropy, electrostatic, HINT potential		MLP			Concu et al. (2009b)
2,276				3D-QSAR			Concu et al. (2009a)
26,632		Global binding descriptors	SVM			1–3	Volkamer et al. (2013)
211,658	Structural	GRAVY				1	Dave & Panchal (2013)
3,095	Sequence	PseAAC, SAAC, GM	ML-kNN				Zou & Xiao (2016)
9,832		FunD, PSSM	OET-kNN			1–2	Shen & Chou (2007)
300,747	Interpro signatures		BR-kNN			1–4	Ferrari et al. (2012)