In Chap. 6 we formalized the MDS structure of micronuclear and intermediate genes through NIDS arrangements and MDS descriptors. In this chapter we shall continue to consider the framework of MDS descriptors. In particular, we shall formalize in this framework the use of the three molecular operations ld, hi, and dlad for gene assembly introduced in Chap. 3. Thus, these operations will be formalized as operations on (rewriting rules for) MDS descriptors. We also prove that this formal model is universal, i.e., every (realistic) MDS descriptor can be reduced to the MDS descriptor (b, e) or its inverse $$\left( {\bar e,\bar b} \right)$$ by these three operations.