Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. 4
        x
      
    
    {\displaystyle 4x}
   = 
  
    
      
        
          y
          
            2
          
        
      
    
    {\displaystyle y^{2}}
   − 
  
    
      
        
          z
          
            2
          
        
      
    
    {\displaystyle z^{2}}
  . A number is a multiple of 4 if its last two digits are a multiple of 4. For example, 1092 is a multiple of 4 because 92 = 4 × 23.
After discovering a second case of ADA deficiency in an immunocompromised patient, ADA deficiency was recognized as the first immunodeficiency disorder. References
Further reading
Adenosine deaminase deficiency - Genetics Home Reference


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