The CAS Registry Mumber 2127-90-4 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 2,1,2 and 7 respectively; the second part has 2 digits, 9 and 0 respectively. Calculate Digit Verification of CAS Registry Number 2127-90: (6*2)+(5*1)+(4*2)+(3*7)+(2*9)+(1*0)=64 64 % 10 = 4 So 2127-90-4 is a valid CAS Registry Number.