Many sensory systems show topographic mapping: the spatial organisation of peripheral receptors is preserved in the brain regions where the sensory information is relayed. One example of such mapping is found in the mouse somatosensory system, where there is clear spatial organisation of the trigeminal ganglion (TG) neurons innervating the whiskers and of their central axons, which in turn project somatotopically to the brainstem. Whisker pattern provides a template for somatotopic map formation, but is it sufficient? Filippo Rijli and colleagues (p. 3704) have devised an elegant neuronal tracing experiment to investigate this, using an Edn1 mutant mouse line that has ectopic whisker rows in the lower jaw. They find that TG neurons innervating these ectopic whiskers acquire molecular characteristics of neurons that normally innervate the whiskers of the upper jaw, suggesting that peripheral input influences the molecular signature of these neurons. However, spatial segregation of these neurons and their axons is poor and topographic mapping fails. Thus, while peripheral signalling influences neuronal identity – and, the authors show, can at later stages cause some repatterning of neuronal targeting – it is not sufficient to instruct topographic mapping in the brain; neuron-intrinsic systems are also essential.