ANP32 proteins have a whip-like shape, with an N-terminal region that is rich in leucine (light gray) and a low-complexity acidic region (LCAR; black). Different species bear different ANP32 proteins. The polymerase of the influenza A virus adapts to the ANP32 proteins that are present in different species so that it can replicate in the cells of a new host. Open circles indicate that an ANP32 protein is unable to support viral replication. In humans, ANP32A, 32B and 32E are present, and the influenza virus can use either ANP32A or ANP32B to support replication (blue circles). Human ANP32C is a pseudogene. In chicken, ANP32A, 32B and 32E are present, and the influenza virus relies exclusively on ANP32A (red circle). Open circles indicate that an ANP32 protein is unable to support viral replication. An exon duplication in ANP32A in chickens (green box) is required for the avian viral polymerase to work. Evolutionary analysis revealed that the avian gene previously identified as ANP32B in chickens is most closely related to ANP32C genes, as indicated by the trees on the outside of the diagram that show species-specific relationships between ANP32 genes. Long et al. and Zhang et al. identified the key residues that must be present in the ANP32 proteins for the viral polymerase to be active: asparagine (N) at site 129 and aspartate (D) at site 130. These amino acids are absent from human ANP32E and from chicken ANP32B and ANP32E.