Sigma says they will not design lenses for MFT only.

Mr. Kazuto Yamaki from Sigma has been interviewed by ePrice Taiwan (google translation here). He said that there is no plan to develop and design lenses purely for the Micro Four Thirds system. They will keep designing them to cover the ASP-C sensor area and adapt the mount for both the NEX and MFT system. Smart move? Don’t know, it certainly keeps the costs of the lens very low.

These are the current Sigma MFT lenses:
Black Sigma 19mm f/2.8 lens at Amazon and Bhphoto.
Silver Sigma 19mm f/2.8 lens at Amazon and Bhphoto.
Black Sigma 30mm f/2.8 lens at Amazon and Bhphoto.
Silver Sigma 30mm f/2.8 lens at Amazon and Bhphoto.
Black Sigma 60mm f/2.8 at BHphoto.
Silver Sigma 60mm f/2.8 at BHphoto.

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Thanks Miroslav!