The CAS Registry Mumber 263390-31-4 includes 9 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 6 digits, 2,6,3,3,9 and 0 respectively; the second part has 2 digits, 3 and 1 respectively. Calculate Digit Verification of CAS Registry Number 263390-31: (8*2)+(7*6)+(6*3)+(5*3)+(4*9)+(3*0)+(2*3)+(1*1)=134 134 % 10 = 4 So 263390-31-4 is a valid CAS Registry Number.