the present invention refers to a transmission unit comprising a transmission input , a transmission output , at least one first main transmission stage and a second main transmission stage connected in series therewith , and a housing encompassing said first and second main transmission stages in common , at least said first main transmission stage being defined by a gear / shaft carrier provided with at least one shaft and at least two gears , and the thus defined first main transmission stage being replaceably inserted in a reception bore of the housing as a modular unit .