The mouse retina has served as an elegant model by which to investigate the molecular mechanisms supporting synaptic specificity and functional circuit connectivity. This is possible due to the well‐characterized catalogue of neuronal types and the well‐described function of the circuits they assemble into. All of these retinal neuronal types are diagramed here in a cross‐sectional view of the retina (rod photoreceptors in turquoise, cone photoreceptors in yellow, horizontal cells in pink, bipolar cells in green, amacrine cells in orange and red, and retinal ganglion cells in blue).

Long bones grow via the production of a transient cartilage scaffold (yellow). Proliferation and differentiation in the cartilage are controlled by three levels of communication: within the cartilage (a), with surrounding tissues (b) and with other organs, likely via a central integrator (c).

Proximity labeling for proteomic profiling. To achieve regional protein labeling, the enzymes are usually fused with a targeting signal peptide or a spatially restricted protein (SP). The enzymes can also be fused with any protein of interest for protein interactome studies. After performing proximity labeling in living cells, the cells are lysed and the tagged endogenous proteins are isolated using steptavidin beads. Small peptides of enriched proteins are generated by trypsin digestion and subsequently ionized for tandem mass spectrometry (MS/MS) analysis. The mass‐to‐charge (m/z) ratio of each peptide and their fragment ions is then used to identify peptide sequence through computational comparison against established databases.