Table 1. Functions of miRNA processing genes and defects caused by abnormal functioning of these genes.

Genes	Functions	Defects caused by abnormal functioning of genes
Drosha	• Vital for the formation of mature miRNA [47]	• 5′ and 3′ nucleotides of the mature miRNA are determined by Drosha cleavage and any defects in Drosha cleavage may lead to changes in the seed sequence of miRNAs [47]
	• miRNA processing by cutting both strands of miRNA forming pre-miRNA product in the presence of DGCR8 [16, 127]
Exp5	• Stabilization of pre-miRNAs [44]	• Knockdown of Exp5 was shown to reduce miRNA expression [43]
	• Transport of pre-miRNAs from the nucleus into the cytoplasm in the presence of Ran and GTP [44]
Dicer	• Maturation of miRNA [16, 26]	• Cell proliferation defects [36]
		• Lack of Dicer in Drosophila germ line stem cells postponed the G1/S phase transition
		• Dicer deletion in hippocampal, mouse and zebrafish initiated problems in nervous system and led to inability of forming mature miRNAs that resulted in variations of brain morphogenesis and differentiation of neurons [128, 129]
		• Complete loss of Dicer1 caused reduced expression of miRNAs and infertile female mice [7, 9]
		• Homozygote Dicer1 germ-line mutant male mice caused decreased male fertility [36]
		• Dicer deficiency led to embryo death in mouse around embryonic day 7.5 [7, 25, 56] and in zebrafish [58]
Ago2	• Component of miRISC [16, 26]	• In the absence of Ago2, oocytes developed to the mature oocytes but with abnormal spindles, and chromosomes were not able to unite properly with reduced miRNA expression levels [57]
	• miRNAs associated with Ago2/RISC complex target mRNAs [13]