CD55, also known as decay-accelerating factor (DAF), is a 70 to 75 kDa member of the regulators of complement/C activation (RCA) family of proteins. It is ubiquitously expressed on cells exposed to plasma complement proteins. Human CD55 is synthesized as a 381 aa precursor that contains a 34 aa signal sequence, a 319 aa mature region and a 28 aa C-terminal prosegment.