In this task, you are given a passage which has a question and the context. You have to generate an answer to the question based on the information present in the context.

Example Input: Context: Nonviral vectors based on polyethylenimine (PEI) usually contain an excess of PEI that is not complexed to DNA. Since unbound PEI contributes to cellular and systemic toxicity, purification of polyplexes from unbound PEI is desirable.', 'Size exclusion chromatography (SEC) was used to purify PEI polyplexes of free PEI. Transfection properties of purified polyplexes and the effect of free PEI on gene delivery were studied in vitro and in vivo after systemic application into mice.', 'SEC did not change the size and zeta-potential of polyplexes. Independent of the amount of PEI used for complex formation, purified PEI polyplexes had the same final PEI nitrogen/DNA phosphate ratio of 2.5. Notably, purified PEI polyplexes demonstrated low cellular and systemic toxicity. High transfection efficiency was achieved with purified polyplexes at high DNA concentrations (8-15 microg/ml). At low DNA concentrations (2-4 microg/ml) gene transfer with purified particles was less efficient than with polyplexes containing free PEI both in vitro and in vivo. Mechanistic studies showed that free PEI partly blocked cellular association of DNA complexes but was essential for the following intracellular gene delivery. Adding free PEI to cells treated with purified particles with a delay of up to 4 h resulted in significantly enhanced transfection efficiency compared with non-purified particles or purified particles without free PEI.\Question: Does purification of polyethylenimine polyplexes highlight the role of free polycations in gene transfer?
Example Output: This study presents an efficient method to remove free PEI from PEI polyplexes by SEC. Our results from transfection experiments demonstrate that free PEI substantially contributes to efficient gene expression but also mediates toxic effects in a dose-dependent manner. Purified polyplexes without free PEI have to be applied at increased concentrations to achieve high transfection levels, but exhibit a greatly improved toxicity profile.

Example Input: Context: This study examines the prevalence and correlates of self-reported premenstrual symptoms among a large, population-based sample of reproductive age, active-duty women.', 'Data were obtained from a combined dataset of two large-scale mail surveys designed to represent the total force. Subjects included in the present study were 6026 active-duty women of all branches of military service stratified by service, paygrade group, race/ethnicity, and location. A multivariate approach is used to evaluate the interrelationships among psychosocial and lifestyle correlates of premenstrual symptoms or pain after controlling for demographic differences in women who reported premenstrual symptoms or pain during the past 3 months (cases) and those who did not (controls).', 'Premenstrual symptoms were reported by nearly 2 of every 3 reproductive age women. Women reporting premenstrual symptoms were more likely to report other symptoms of menstrual dysfunction, two or more current medical conditions, migraines, and healthcare provider visits in the past year. After controlling for the protective effects of taking Depo-Provera (Upjohn, Kalamazoo, MI) and ever being pregnant, younger age, trying to lose weight, heavier drinking, poorer self-perceived health, and overall job stress were the most significant predictors of premenstrual symptoms. The greatest risk factor was a high level of job stress, with an almost 3-fold increase in risk relative to those without symptoms.\Question: Does psychosocial and lifestyle correlate of premenstrual symptoms among military women?
Example Output: Work stress may mediate the relationship among depression and premenstrual symptoms. Further research is needed to elucidate the biological interrelationships among work stress, hormonal function, and premenstrual symptomatology.

Example Input: Context: Static magnetic fields (SMF) have been widely used in research, medicine and industry. Since zinc and copper play an important role in biological systems, we studied the effects of the subchronic continuous SMF exposure on their distribution in murine tissues.', 'For 30 days, mice were exposed to inhomogeneous, vertical, downward or upward oriented SMF of 1mT averaged intensity with spatial gradient in vertical direction.', 'SMF decreased the amount of copper and zinc in liver. In brain, zinc levels were increased and copper levels were decreased. In spleen, zinc content was reduced, while copper amount remained unchanged.\Question: Does subchronic exposure to static magnetic field differently affect zinc and copper content in murine organs?
Example Output:
Subchronic exposure to SMF differently affected copper and zinc content in examined organs, and the changes were more pronounced for the downward oriented field. The outcome could be attributed to the protective, rather than the harmful effect of SMF.