At a minimum there were two additional symbols: "N" for uNknown, where the raw sensor information - which should be a series of nice Gaussians[1 top] - was too

At a minimum there were two additional symbols: "N" for uNknown, where the raw sensor information - which should be a series of nice Gaussians[1 top] - was too ambiguous/low-quality for a useful basecall, but the presence of something can be inferred from the timing information[1 bottom, with "N"s in some bases] An additional unused bit pattern was reserved for future emergency additions. It used the longest bit pattern so future additions wouldn't be balanced properly, but I wanted something simple, guaranteed binary-compatibility, that could handle a sudden change in requirements. In some situations, a variation was used that included a few symbols for partial basecalls like "unknown purine" (A or G) and "unknown pyrimidine" (C or T). [1] http://image.slidesharecdn.com/binoinfo-8-141021112354-conversion-gate02/95/7-11-638.jpg http://image.slidesharecdn.com/binoinfo-8-141021112354-conve...