PMOS primer assembly scheme. The PMOS system consists of 256 extendable oligonucleotides (EO's) and 512 template oligonucleotides (TO's) distributed into two 384 well microtitre plates. In each case defined sequence positions for the individual library members are designated X; and in the TO degenerate positions are designated N. To generate each composite 11-mer, the required EO and TO is selected from the library and transferred to a reaction vessel. When combined the EO and TO associate via their complementary clamp (blue) and catch (green) regions. In the presence of DNA polymerase and nucleotides (orange), such as in sequencing chemistry, the EO is extended along the overlapping template (TO). Extension culminates with a single base extension (dA) beyond the limit of the template. In subsequent thermal cycles the sequence specific 11-mer derived from the extended EO can function as a template specific primer in sequencing and other applications. At the bottom of the diagram the composite 11-mer primer is shown annealed to template (yellow) as it finds deployment in the initiation of an extension reaction such as in DNA sequencing.