The CAS Registry Mumber 1609-27-4 includes 7 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 4 digits, 1,6,0 and 9 respectively; the second part has 2 digits, 2 and 7 respectively. Calculate Digit Verification of CAS Registry Number 1609-27: (6*1)+(5*6)+(4*0)+(3*9)+(2*2)+(1*7)=74 74 % 10 = 4 So 1609-27-4 is a valid CAS Registry Number.