The FMO, or Fluorescence Minus One, is a type of control used to properly interpret flow cytometry data. It is used to identify and gate cells in the context of data spread due to the multiple fluorochromes in a given panel.
An FMO control contains all the flurochromes in a panel, except for the one that is being measured. For example, in the four color panel, there would be four separate FMO controls, as shown in the table below.
The FMO control ensures that the any spread of the fluorochromes into the channel of interest is properly identified. The proper use is illustrated below. The image is courtesy of Mario Roederer.
As can be seen in this figure, the difference between the isotype bound and the FMO bound represents the spread of the data due to the fluorescence spread in the PE channel.