  We prove that the abstract Tile Assembly Model (aTAM) of nanoscale
self-assembly is intrinsically universal. This means that there is a single
tile assembly system U that, with proper initialization, simulates any tile
assembly system T. The simulation is "intrinsic" in the sense that the
self-assembly process carried out by U is exactly that carried out by T, with
each tile of T represented by an m x m "supertile" of U. Our construction works
for the full aTAM at any temperature, and it faithfully simulates the
deterministic or nondeterministic behavior of each T.
  Our construction succeeds by solving an analog of the cell differentiation
problem in developmental biology: Each supertile of U, starting with those in
the seed assembly, carries the "genome" of the simulated system T. At each
location of a potential supertile in the self-assembly of U, a decision is made
whether and how to express this genome, i.e., whether to generate a supertile
and, if so, which tile of T it will represent. This decision must be achieved
using asynchronous communication under incomplete information, but it achieves
the correct global outcome(s).
