Modeling the RNA pool generation process using mixing matrices and analysis of pool structural distributions using tree graphs. This has been redrawn in light of figure presented in . The mixing matrix applied to any starting sequence specifies the mutation rates for all nucleotide bases. The matrix elements of each row represent nucleotide base (A, C, G, and U) composition in a vialor synthesis port. Mixing matrices and starting sequences can be optimized to yield target structured pools.