Mammalian LINE-1 (L1) elements belong to the superfamily of autonomously 
replicating retrotransposable elements that lack the long terminal repeated 
(LTR) sequences typical of retroviruses and retroviral-like retrotransposons. 
The non-LTR superfamily is very ancient and L1-like elements are ubiquitous in 
nature, having been found in plants, fungi, invertebrates, and various 
vertebrate classes from fish to mammals. L1 elements have been replicating and 
evolving in mammals for at least the past 100 million years and now constitute 
20% or more of some mammalian genomes. Therefore, L1 elements presumably have 
had a profound, perhaps defining, effect on the evolution, structure, and 
function of mammalian genomes. L1 elements contain regulatory signals and encode 
two proteins: one is an RNA-binding protein and the second one presumably 
functions as an integrase-replicase, because it has both endonuclease and 
reverse transcriptase activities. This work reviews the structure and biological 
properties of L1 elements, including their regulation, replication, evolution, 
and interaction with their mammalian hosts. Although each of these processes is 
incompletely understood, what is known indicates that they represent challenging 
and fascinating biological phenomena, the resolution of which will be essential 
for fully understanding the biology of mammals.