The nucleus of eukaryotic cells is separated from the cytoplasm by a double membrane, the nuclear envelope, which is derived from the endoplasmic reticulum. Nuclear pores, formed by the nuclear pore complex (NPC), allow the transport of molecules between the nucleus and cytoplasm. These pores are large protein structures that regulate the movement of proteins and nucleic acids, enabling processes such as DNA replication, transcription, and cell cycle regulation. The transport of molecules through the NPC is controlled by specific signals, such as nuclear localization signals (NLSs) and nuclear export signals (NESs), which are recognized by soluble carrier proteins, including importins and exportins. Importins, such as importin α and β, bind to NLSs and facilitate nuclear import, while exportins, like Crm1, recognize NESs and promote nuclear export. The Ran GTP/GDP cycle plays a crucial role in regulating the direction and efficiency of transport. RanGEF and RanGAP control the GTP/GDP state of Ran, which in turn influences the binding of importins and exportins to their cargo. The structure of the NPC and the interactions between cargo and transport proteins are essential for the selective and efficient transport of molecules across the nuclear envelope. The study of nucleocytoplasmic transport has revealed the complexity of the process, including the involvement of various carriers, cofactors, and the dynamic regulation of transport signals. Understanding these mechanisms is crucial for elucidating the functions of the nucleus and the regulation of cellular processes.The nucleus of eukaryotic cells is separated from the cytoplasm by a double membrane, the nuclear envelope, which is derived from the endoplasmic reticulum. Nuclear pores, formed by the nuclear pore complex (NPC), allow the transport of molecules between the nucleus and cytoplasm. These pores are large protein structures that regulate the movement of proteins and nucleic acids, enabling processes such as DNA replication, transcription, and cell cycle regulation. The transport of molecules through the NPC is controlled by specific signals, such as nuclear localization signals (NLSs) and nuclear export signals (NESs), which are recognized by soluble carrier proteins, including importins and exportins. Importins, such as importin α and β, bind to NLSs and facilitate nuclear import, while exportins, like Crm1, recognize NESs and promote nuclear export. The Ran GTP/GDP cycle plays a crucial role in regulating the direction and efficiency of transport. RanGEF and RanGAP control the GTP/GDP state of Ran, which in turn influences the binding of importins and exportins to their cargo. The structure of the NPC and the interactions between cargo and transport proteins are essential for the selective and efficient transport of molecules across the nuclear envelope. The study of nucleocytoplasmic transport has revealed the complexity of the process, including the involvement of various carriers, cofactors, and the dynamic regulation of transport signals. Understanding these mechanisms is crucial for elucidating the functions of the nucleus and the regulation of cellular processes.