The genetic code evolved in two distinct phases. First, the 'canonical' cod
e emerged before the last universal ancestor; subsequently this code diverg
ed in numerous nuclear and organelle lineages. Here, we examine the distrib
ution and causes of these secondary deviations from the canonical genetic c
ode. The majority of non-standard codes arise from alterations in the tRNA,
with most occurring by post-transcriptional modifications, such as base mo
dification or RNA editing, rather than by substitutions within tRNA anticod
ons.