As your brain is growing, neurons branch and form connections with neurons nearby. As they’re doing this, they need to know not to connect to themselves, which would form a tangled mess. The protocadherins let them practice self-avoidance. Like a barcode, each neuron randomly expresses a combination of these protocadherins, and knows not to connect when it sees that same barcode.
Click here to see all 8644679 letters of Day 120 with protocadherin 1 (PCDH1) underlined.