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Figure 1: Characteristics of the tumor microenvironment or niche, and validated microRNAs and their target genes in the microenvironment. The tumor microenvironment is a complex scaffold of extracellular matrix and various cell types. In addition to tumor cells, an assemblage of distinct cell types, including endothelial cells, fibroblasts, and immune cells, as well as extracellular matrix molecules, contribute to tumor growth and progression. Tumor tissue is characterized by self-sufficiency in growth signals, which provides replicative immortality, and enables angiogenesis, cell invasion, and metastasis. The different cell types in the tumor can produce growth and antigrowth signals and respond to stimuli secreted by other cells. This creates a favorable microenvironment for tumor growth and spread. The influence of microRNAs on the tumor microenvironment is related to cancer progression. MicroRNAs are a critical component of the tumor microenvironment involved in invasion and metastasis of cancer cells. The deregulated expression of microRNAs in the tumor microenvironment could contribute to cancer proliferation, invasion, and metastasis. See text for more details.