The CAS Registry Mumber 105999-21-1 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 1,0,5,9,9 and 9 respectively; the second part has 2 digits, 2 and 1 respectively. Calculate Digit Verification of CAS Registry Number 105999-21: (8*1)+(7*0)+(6*5)+(5*9)+(4*9)+(3*9)+(2*2)+(1*1)=151 151 % 10 = 1 So 105999-21-1 is a valid CAS Registry Number.