The Baltimore classification clusters viruses into families depending on their type of genome.
Unlike LUCA for cellular organism, there is no presumed common ancestor for viruses. The present virus classification comprises seven trees of life, classified under the Baltimore system.
David Baltimore (Nobel prize 1975) proposed a classification based on the genome present in virions in a publication Expression of animal virus genome.

Minimum viral enzymes:

	Group I, dsDNA	Group II ssDNA	Group III dsRNA	Group IV ssRNA+	Group V ssRNA-	Group VI RT RNA	Group VII RT DNA
General	[DNApol, transcriptase, Helicase (if cytoplasmic)]	Rep endonuclease	RdRp Helicase	RdRp Helicase	RdRp	RTase RNAseH Integrase	RTase RNAseH
Eukaryotic host	[capping (if cytoplasmic)]		capping	capping	capping