TABLE 1.

Comparison of properties of mammalian autonomous retroelements

Property or characteristic	Non-LTR	LTR
% of Mammalian genome	∼20	∼8
No. of families	One major (LINE-1); two minor (LINE-2 and 3)	Many
Life history	One origin or introduction of LINE-1 into the mammalian genome; few extinctions	Many independent introductions into the genome from extant retroviruses; many extinctions
Period when transposition occurs for a group	Long term (>100 Myr)	Short to medium term (usually <20 Myr)
Horizontal transfer	None known	Sometimes
No. of potentially active members per group	100 to 1,000s	Usually few
Divergence between active members	Usually low; one to a few lineages per host species	Lineage number may increase with age; many distantly related groups in the same host species
Ability to move other elements in trans	Yes: SINEs, pseudogenes	Yes: nonautonomous ERVs
Modulate gene expression	Yes	Yes