The Influenza A Virus Replication Cycle: A Comprehensive Review Toby Carter and Munir Iqbal summarize the key steps in the influenza A virus (IAV) replication cycle, highlighting areas requiring further research. IAV is a major cause of influenza, infecting up to a billion people annually and causing severe outbreaks in birds. A highly pathogenic, human-transmissible IAV subtype could lead to a pandemic. Current antiviral drugs target few aspects of IAV replication and may become ineffective due to resistance. New antivirals targeting other parts of the replication cycle are needed to combat annual epidemics. IAV virions have spherical or filamentous shapes and enter cells via endocytosis, primarily clathrin-mediated. After endocytosis, the virion fuses with late endosomes, releasing the viral ribonucleoproteins (vRNPs) into the cytosol. The vRNPs are imported into the nucleus via the classical importin pathway. The viral polymerase transcribes and replicates the viral genome, producing positive-sense viral mRNA and complementary RNA (cRNA). The cRNA is used to form complementary RNP (cRNP), which then undergoes secondary replication to produce progeny vRNPs. The progeny vRNPs are exported from the nucleus and trafficked to the plasma membrane, forming a bundle with all eight segments. The fully assembled genome is loaded into the budding virion, which is released from the cell surface by the sialidase activity of neuraminidase (NA). The viral transmembrane proteins induce budding, with the budozone membrane incorporated into the growing virion. The matrix protein M1 oligomerises into a matrix layer beneath the membrane, and the fully assembled genome bundle is recruited into the tip of the budding virion. The review details the structure and function of key IAV proteins, including the polymerase, haemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), and matrix proteins M1 and M2. HA mediates cell entry by binding sialic acid, while NA facilitates viral release from the cell surface. The polymerase is a heterotrimer containing PB2, PB1, and PA, which binds the v/cRNA 3' and 5' termini and transcribes and replicates the viral genome. The vRNA promoter is highly conserved and forms a duplex with multiple inter- and intra-strand base pairs. The review also discusses the binding and endocytosis of IAV, the fusion of the viral and endosomal membranes, and the nuclear import of vRNPs. The viral genome is replicated in the nucleus, and the vRNPs are released into the cytosol. The review highlights the importance of understanding the IAV replication cycle to develop new antiviral strategies and prevent future pandemics.The Influenza A Virus Replication Cycle: A Comprehensive Review Toby Carter and Munir Iqbal summarize the key steps in the influenza A virus (IAV) replication cycle, highlighting areas requiring further research. IAV is a major cause of influenza, infecting up to a billion people annually and causing severe outbreaks in birds. A highly pathogenic, human-transmissible IAV subtype could lead to a pandemic. Current antiviral drugs target few aspects of IAV replication and may become ineffective due to resistance. New antivirals targeting other parts of the replication cycle are needed to combat annual epidemics. IAV virions have spherical or filamentous shapes and enter cells via endocytosis, primarily clathrin-mediated. After endocytosis, the virion fuses with late endosomes, releasing the viral ribonucleoproteins (vRNPs) into the cytosol. The vRNPs are imported into the nucleus via the classical importin pathway. The viral polymerase transcribes and replicates the viral genome, producing positive-sense viral mRNA and complementary RNA (cRNA). The cRNA is used to form complementary RNP (cRNP), which then undergoes secondary replication to produce progeny vRNPs. The progeny vRNPs are exported from the nucleus and trafficked to the plasma membrane, forming a bundle with all eight segments. The fully assembled genome is loaded into the budding virion, which is released from the cell surface by the sialidase activity of neuraminidase (NA). The viral transmembrane proteins induce budding, with the budozone membrane incorporated into the growing virion. The matrix protein M1 oligomerises into a matrix layer beneath the membrane, and the fully assembled genome bundle is recruited into the tip of the budding virion. The review details the structure and function of key IAV proteins, including the polymerase, haemagglutinin (HA), nucleoprotein (NP), neuraminidase (NA), and matrix proteins M1 and M2. HA mediates cell entry by binding sialic acid, while NA facilitates viral release from the cell surface. The polymerase is a heterotrimer containing PB2, PB1, and PA, which binds the v/cRNA 3' and 5' termini and transcribes and replicates the viral genome. The vRNA promoter is highly conserved and forms a duplex with multiple inter- and intra-strand base pairs. The review also discusses the binding and endocytosis of IAV, the fusion of the viral and endosomal membranes, and the nuclear import of vRNPs. The viral genome is replicated in the nucleus, and the vRNPs are released into the cytosol. The review highlights the importance of understanding the IAV replication cycle to develop new antiviral strategies and prevent future pandemics.