Mg is a lightweight structural material with a good specific strength. Unfortunately, it lacks good room temperature formability and therefore a wider commercial use of Mg is hindered. The preferred basal slip and strong basal-type texture were found to be the main reasons for the poor room temperature formability. For basal slip the von Mises’ criterion is not fulfilled and only two independent deformation modes are available for a shape change instead of the needed five independent deformation modes for an arbitrary shape change. Recent experimental and simulative studies tried to manipulate the alloy system to activate more slip systems without changing strength and work hardening properties. Alloys containing Y and rare-earth elements showed a highly increased room-temperature ductility which could be traced back to the increased activity of < c+a > dislocation slip with TEM measurements and ab initio calculations. Usually, < c+a > dislocation slip is not active in pure Mg and other Mg alloys. The activation of shear modes out of the basal plane is correlated to the formation of I₁ intrinsic stacking faults (SFI₁) which is related to the I₁ intrinsic stacking fault energy (I₁ SFE). It is assumed that Y and rare-earth elements lower the I₁ SFE allowing the nucleation of < c+a > dislocation through sessile SFI₁. As the SFI₁ is bound by pyramidal partial dislocations this configuration enables dislocations on pyramidal planes. Since Y and rare-earth elements are costly further research has been carried out to identify similar acting alloy systems. For this purpose, the parameter I₁ SFE has been used and the system Mg-Al-Ca was identified to have a similar I₁ SFE. In experiments the system indeed showed the expected increased room temperature ductility. Further research of the effects of the chemical composition on the ductilisation of this alloy has now been performed.