Most animal species exhibit left-right asymmetry in their body plans a
nd show a strong bias for one handedness over the other. The mechanism
of handedness choice, recognized as an intriguing problem over a cent
ury ago, is still a mystery. However, from recent advances in understa
nding when and how asymmetry arises in both invertebrates and vertebra
tes, developmental pathways for establishment and maintenance of left-
right differences are beginning to take shape, and speculations can be
made on the initial choice mechanism.