L-Mount, making it possible to use alternative lenses that meet the L-Mount specifications of partners (Leica and Sigma)
From the official press release - the key features of the newly developed cameras, the LUMIX S1R and the S1, are as follows:
Expressive capabilities with high definition and high level of expression achieved with the newly-developed 35 mm full-frame image sensor and image processing engine. (Effective pixels: approximately 47M for the S1R and 24M for the S1)
World's first*1 support for 4K 60p/50p video recording in a full-frame Digital Single Lens Mirrorless camera.
World's first*1 full-frame camera equipped with Dual I.S. (Image Stabilization), enabling handheld shots for dark or distant scenes that would previously have required a tripod or other equipment.
A double slot for XQD memory cards and SD memory cards, the first for Panasonic; and a rugged triaxial tilt LCD, emphasizing ease of use and supporting professional photography and videography.
Leica Camera's L-Mount, making it possible to use interchangeable lenses that meet the L-Mount specifications of the partners*4 Leica Camera and Sigma. Expressiveness is further enhanced by increasing options for interchangeable lenses.