Figure 1.

The structure of FCS files. The HEADER is a fixed size (58 bytes), ASCII encoded segment, which always occurs at the beginning of an FCS file. The first six bytes of each header are occupied by FCS' version (e.g. "FCS3.0"). The rest of the header describes the location of the other segments in the data files ('text seg. begin', 'text seg. end', 'data seg. begin', 'data seg. end', 'analysis seg begin' and 'analysis seg. end'). The HEADER may be extended beyond 58 bytes, if custom segments exist in a FCS file. The TEXT segment is divided in 'primary' and 'supplemental' sub-segments, containing a series of ASCII encoded keyword-value pairs that describe various aspects of the data and the conditions under which the experiment took place. The DATA segment contains the raw data, consisting of a number of light-intensity measurements (varying from 6 to 34, in most cases) for each passing cell, depending on the particular cytometer's technology. The raw data are stored in one of four allowed formats ('integer', 'floating point', 'double precision floating point' or 'ASCII'). The mode, format, bit length and all other necessary information needed to extract the raw data from an FCS file are stored in the TEXT segment.