Leica, Sigma, and Panasonic have aligned for future L-mount development, a system for mirrorless APS-C and full frame format cameras. First announced in 2014 with the Leica T (and then again in 2015 with its full frame Leica S)L, the L-mount has a 51.6mm inner diameter and a 20mm flange focal length.
A new L-mount website has been dedicated to its development and features a Q&A worth reading. Currently, L-mount is on the Leica SL, Leica TL2, and Leica CL cameras as well as Panasonic's new full frame mirrorless S1R and S1 cameras. Sigma is expected to release L-mount tech as well.
Be sure to bookmark the new L-mount website here.