The use of mixtures ensures the very high efficiency of the process. Both reactants could be mixtures but for practical reasons the split-mix procedure is used: one mixture is divided into portions that are coupled with the BBs. The mixtures are so important that there is no combinatorial library without using a mixture in the synthesis, and if a mixture is used in a process inevitably combinatorial library forms. ″ Components of the libraries need to be present in nearly equal molar quantities. In order to achieve this as closely as possible the mixtures are divided into equal portions and after pooling a thorough mixing is needed. ″ Since the structure of components is unknown deconvolution methods need to be used in screening. For this reason, encoding methods had been developed. Coding molecules are attached to the beads of the solid support that record the coupled BBs and their sequence. One of these methods is encoding by DNA oligomers. ″ It is a remarkable feature of combinatorial libraries that the whole compound mixture can be screened in a single process. Since both the synthesis and screening are very efficient procedures the use of combinatorial libraries in pharmaceutical research leads to enormous savings. In solid phase combinatorial synthesis only a single compound forms in each bead. For this reason, the number of components in the library can't exceed the number of beads of the solid support. This means that the number of components in such libraries is limited. This restraint was eliminated by Harbury and Halpin. In their synthesis of DELs, the solid support is omitted and BBs are attached directly to the encoding DNA oligomers. This new approach helps to increase practically unlimitedly the number of components of DNA encoded combinatorial libraries (DECLs). Split-&-Pool DNA Encoding In order to apply combinatorial chemistry for the synthesis of DNA-encoded chemical libraries, a Split-&-Pool approach was pursued. Initially a set of unique DNA-oligonucleotides (n) each containing a specific coding sequence is chemically conjugated to a corresponding set of small organic molecules. Consequently, the oligonucleotide-conjugate compounds are mixed ("Pool") and divided ("Split") into a number of groups (m). In appropriate conditions a second set of building blocks (m) are coupled to the first one and a further oligonucleotide which is coding for the second modification is enzymatically introduced before mixing again. This “split-&-pool” steps can be iterated a number of times (r) increasing at