Effective intracellular immunity targets incoming viruses. Whereas the ecotropic murine leukemia virus (MLV) (represented as a viral particle in blue) encounters Fv4 and Fv1 blocks, other retroviruses such as HIV-1, SIV and EIAV (represented as a viral particle in brown) encounter TRIM5α and cytidine deaminase blocks. Fv4 prevents the entry by ecotropic MLV by sequestering the viral receptor from the cell surface. Fv1 targets MLV CA and stops the nuclear import of the viral preintegration complex (PIC). TRIM5α also targets retroviral CA and blocks uncoating. hA3B, hA3F and hA3G deaminate cytidines on newly synthesized retroviral cDNA and disrupt viral replication. Capital red letters highlight the points of inhibition. Viral structural components, nucleic acids, RNA and DNA, and intracellular events are represented in different colors.