The CAS Registry Mumber 321-53-9 includes 6 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 3 digits, 3,2 and 1 respectively; the second part has 2 digits, 5 and 3 respectively. Calculate Digit Verification of CAS Registry Number 321-53: (5*3)+(4*2)+(3*1)+(2*5)+(1*3)=39 39 % 10 = 9 So 321-53-9 is a valid CAS Registry Number.