A variety of GPI-linked cell-surface glycoproteins are composed of one or more copies of a conserved domain of about 100 amino-acid residues [1,2]. These proteins are:

• Urokinase plasminogen activator surface receptor u-PAR, which binds to u-PA and is responsible for its proteolysis-independent signal transduction activation effects.
• The mouse family of Ly-6 T-cell antigens (Ly-6A, -6C, -6E, -6F and -6G).
• CD59 (MACIF or MIRL), an inhibitor of the complement membrane attack complex action.
• Herpesvirus saimiri CD59-related protein (gene 15).
• Mouse thymocyte B cell antigen (ThB).
• Squid glycoprotein Sgp-2.

U-PAR contains three tandem copies of the domain, while all the others are made up of a single domain.

As shown in the following schematic, this conserved domain contains 10 cysteine residues involved in five disulfide bonds - in u-PAR, the first copy of the domain lacks the fourth disulfide bond.

     +------+     +------------------------+                    +---+
     |      |     |                        |                    |   |
 xCxxCxxxxxxCxxxxxCxxxxxCxxxxxxxxxxxxxxxxxxCxxxxCxxxxxxxxxxxxxxCCxxxCxxxxxxxx
  |                     |                       |              |
  +---------------------+                       +--------------+

'C': conserved cysteine involved in a disulfide bond.

The signature pattern that detects this domain starts towards its extremity and ends at the sixth cysteine.