The CAS Registry Mumber 56355-34-1 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 5,6,3,5 and 5 respectively; the second part has 2 digits, 3 and 4 respectively. Calculate Digit Verification of CAS Registry Number 56355-34: (7*5)+(6*6)+(5*3)+(4*5)+(3*5)+(2*3)+(1*4)=131 131 % 10 = 1 So 56355-34-1 is a valid CAS Registry Number.