Smaug is a RNA-binding protein in Drosophila that helps in maternal to zygotic transition (MZT).[3][4] The protein is named after the fictional character Smaug, the dragon in J.R.R. Tolkien's 1937 novel The Hobbit.[5] The MZT ends with the midblastula transition (MBT), which is defined as the first developmental event in Drosophila that depends on zygotic mRNA.[6] In Drosophila, the initial developmental events are controlled by maternal mRNAs like Hsp83, nanos, string, Pgc, and cyclin B mRNA.[7][8][9][1][10] Degradation of these mRNAs, which is expected to terminate maternal control and enable zygotic control of embryogenesis, happens at interphase of nuclear division cycle 14.[5][11] During this transition smaug protein targets the maternal mRNA for destruction using miRs. Thus activating the zygotic genes. Smaug is expected to play a role in expression of three miRNAs – miR-3, miR-6, miR-309 and miR-286 during MZT in Drosophila.[6] Among them smaug dependent expression of miR-309 is needed for destabilization of 410 maternal mRNAs.[12] In smaug mutants almost 85% of maternal mRNA is found to be stable. Smaug also binds to 3′ untranslated region (UTR) elements known as SMG response elements (SREs) on nanos mRNA[13][14] and represses its expression by recruiting a protein called Cup (an eIF4E-binding protein that blocks the binding of eIF4G to eIF4E).[15][16] There after it recruits deadenylation complex CCR4-Not on to the nanos mRNA which leads to deadenylation and subsequent decay of the mRNA.[17] It is also found to be involved in degradation and repression of maternal Hsp83 mRNA by recruiting CCR4/POP2/NOT deadenylase to the mRNA.[1] The human Smaug protein homologs are SAMD4A and SAMD4B.