Background: microRNAs (miRs) have emerged as key regulators of both normal and pathologic phenotypes, including cancer.Functional activities and biogenesis of miRs are heavily regulated.Dysregulation of physiologic miR activity and biogenesis has been shown to play an important role in tumor initiation and progression.However, few modulators of miR activity and biogenesis have been characterized, and both the extent and relevance of their role in controlling normal cell physiology and pathogenesis are poorly understood.Methods: Systematic approaches were exploited to dissect microRNA-related posttranscriptional regulatory networks that are associated with human cancers.Specifically, by analyzing mRNA and miR expression profiles of a large set of glioblastoma patient samples, miR activity and biogenesis regulations have been studied in a genome-wide scale.Results: First, genome-wide screening of miR activity regulators uncovered a previously-uncharacterized layer of post-transcriptional regulation, involving~7,000 RNAs that act as miR "sponges" and participate in more than 248,000 miR program-mediated (mPM) interactions.This regulation layer also includes 148 genes that regulate individual miR activity on its targets through alternative, nonsponge mechanisms.These mPM interactions as a network mediate crosstalk between canonical oncogenic pathways and connect genetically distant genes, which was experimentally validated in tumor cell lines.The mPM interactions also provide mechanistic rationales for regulation of key genes in cancer, and largely explain missing genetic variations in various patient groups.mPM interaction networks were also reconstructed for breast, prostate, and ovarian cancer.Second, hundreds of novel regulators of miR biogenesis were discovered through a genome-wide screening using the same dataset as above.A subset of these regulators have been experimentally validated via high-, mid-, and low-throughput profiling of primary, precursor, and mature miRs in addition to PAR-CLIP analyses.Some of these regulators control key disease-driving genes by post-transcriptionally regulating the abundance of miRs, and were differentially expressed across subtypes of glioblastoma.Conclusions: Systematic studies of miR-related regulations revealed a miR-mediated, post-transcriptional regulation layer of unsuspected magnitude, the mPM network.Together with the miR biogenesis regulation, these miR-related regulatory networks provide novel mechanisms of tumor pathogenesis and progression .