Sentence:  One hour prior to transfection, the culture medium was replaced with serum-free RPMI medium. 0.4 μg of DNA (pNN9 or DNA <WSD>ministring</WSD>), diluted in 50 μl of serum-free OptiMEM culture medium, was mixed with different aliquots of 1.5 mM 16-3-16 gemini surfactant solution to yield N + /P − charge ratios of 3:1 and 5:1. 
Meaning of the word marked with <WSD> tags:  A short linear covalently-closed DNA vector