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Javier Mercapide; Esteban Santiago; Elena Alberdi; Juan J. Martinez–Irujo. (1999). Contribution of phosphodiesterase isoenzymes and cyclic nucleotide efflux to the regulation of cyclic GMP levels in aortic smooth muscle cells. , 58(10), 1675–1683. doi:10.1016/s0006-2952(99)00252-x 
An interview with Sci-Hub Founder Alexandra Elbakyan,Who exactly should pay for academic research