Complex sentence: Because nucleases that cleave the phosphodiester linkage in DNA are expressed in most every cell, unmodified DNA molecules are generally degraded before their reach their targets, and so antisense drug candidate molecules are generally modified during the drug discovery phase of their development.
Split into simpler sentences: Because nucleases that cleave the phosphodiester linkage in DNA are expressed in almost every cell, unmodified DNA molecules are generally degraded before they reach their targets. Therefore, antisense drug candidate molecules are generally modified during the drug discovery phase of their development.