A cassette unit (1; 101; 201; 301; 501; 601) for use with an auto-injector having an electlically powered drive unit, said cassette unit comprising: a cassette unit housing (20; 120; 220; 320; 520; 620) defining a cassette unit housing cavity and a needle projection aperture; said cassette unit housing cavity arranged for receipt of a syringe (10; 110; 210; 310; 510) comprising: a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation; a hollow needle (14; 114; 214; 314; 514; 614) at a front end of said barrel, said hollow needle defining a needle tip (15; 115; 215; 315; 515; 615) for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and a removable cap (50; 150; 250; 350; 550; 650) that in a capping position fits over and thereby, acts such as to close off, the needle projection aperture, said removable cap provided with one or more first engagement features (52; 152; 252; 352) arranged for selectively engaging one or more second engagement features (226; 326) of the cassette unit housing when the cap is in the capping position; a cap lock feature (40, 42; 140, 142; 240; 340) movable from a first cap locking position in which it prevents removal of the removable cap from said capping position with the cassette unit to a second cap non-locking position in which it no longer prevents such cap removal, characterised in that the cap lock feature includes one or more locking features, which in the first cap locking position act such as to prevent movement of one or more of the first engagement features of the removable cap and /or the second engagement features of the cassette unit housing. a cassette unit housing (20; 120; 220; 320; 520; 620) defining a cassette unit housing cavity and a needle projection aperture; said cassette unit housing cavity arranged for receipt of a syringe (10; 110; 210; 310; 510) comprising: a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation; a hollow needle (14; 114; 214; 314; 514; 614) at a front end of said barrel, said hollow needle defining a needle tip (15; 115; 215; 315; 515; 615) for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and a barrel (12; 112; 212; 312; 512) for containing a volume of a liquid drug formulation; a hollow needle (14; 114; 214; 314; 514; 614) at a front end of said barrel, said hollow needle defining a needle tip (15; 115; 215; 315; 515; 615) for dispensing of said liquid drug formulation; and a plunger (18; 118; 218; 318) that is axially movable within the barrel; and a removable cap (50; 150; 250; 350; 550; 650) that in a capping position fits over and thereby, acts such as to close off, the needle projection aperture, said removable cap provided with one or more first engagement features (52; 152; 252; 352) arranged for selectively engaging one or more second engagement features (226; 326) of the cassette unit housing when the cap is in the capping position; a cap lock feature (40, 42; 140, 142; 240; 340) movable from a first cap locking position in which it prevents removal of the removable cap from said capping position with the cassette unit to a second cap non-locking position in which it no longer prevents such cap removal, characterised in that the cap lock feature includes one or more locking features, which in the first cap locking position act such as to prevent movement of one or more of the first engagement features of the removable cap and /or the second engagement features of the cassette unit housing.