The 'master' human mobile element, the L1 retrotransposon, has come of
age as a biological entity. Knowledge of how it retrotransposes in vi
vo, how its proteins act to retrotranspose other poly A elements and t
he extent of its role in shaping the human genome should emerge rapidl
y over the next few years. We review the impact of retrotransposons an
d how new insight is likely to lead to important practical application
s for these intriguing mobile elements.