The CAS Registry Mumber 1209-99-0 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,2,0 and 9 respectively; the second part has 2 digits, 9 and 9 respectively. Calculate Digit Verification of CAS Registry Number 1209-99: (6*1)+(5*2)+(4*0)+(3*9)+(2*9)+(1*9)=70 70 % 10 = 0 So 1209-99-0 is a valid CAS Registry Number.