A chimera brain could have serious consequences. For instance, we know that the arrangement of different brain regions can be crucial for its function – but the presence of foreign tissue, being directed by different genes carrying a different blueprint, may throw that intricate design into disarray. This may explain, for instance, why twins are less likely to be right-handed – a simple trait that normally relies on the relative organisation of the right and the left hemispheres. Perhaps chimerism has upset the balance.