The CAS Registry Mumber 99391-13-6 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 9,9,3,9 and 1 respectively; the second part has 2 digits, 1 and 3 respectively. Calculate Digit Verification of CAS Registry Number 99391-13: (7*9)+(6*9)+(5*3)+(4*9)+(3*1)+(2*1)+(1*3)=176 176 % 10 = 6 So 99391-13-6 is a valid CAS Registry Number.