Adenosine regulates many processes in the human body by its A1, A2A, A2B and A3 receptors and influences an embryonal development of some organs. Adenosine deaminase (ADA) is the most important enzyme of adenosine metabolism from the pathological viewpoint. A loss of its adenosine deaminase activity results in severe combined immunodeficiency disease (SCID). CECR1 is the next enzyme with proposed adenosine deaminase activity and it is supposed to be the cause of heart defects in Cat Eye Syndrome. The next homologues of ADA were revealed recently. Their role in human body could help to clarify a base of SCID and could result to complex clarification of the adenosine metabolism in the human body.

        Key words: adenosine, adenosine deaminase, SCID, CECR1.