The clinical features of infantile Tay–Sachs disease seen in our set of
patients were consistent with the defined phenotype, except for the
presence of prominentMongolian spots on the back even beyond infancy
in two patients. However,Mongolian spots are also reported in association
withmany of the patientswith other LSDs [1,16] and in our case
it could be an accidental finding. The enzyme activitywas in accordance
with the previous observations of infantile TSD cases [17,18]. However,
MUG substrate is cleaved by all three isozymes and therefore is used to
measure total-Hex activity. MUGS substrate is cleaved primarily by the
α-subunit containing isozymes (Hex-A and S). However, the β active
site can slowly hydrolyze this substrate with MUG/MUGS ratios of
~300:1 for Hex-B and 3.7:1 for Hex-A [19]. Thus each of the %-residual
Hex-A values reported in Table 1 is multiplied by 3.7 and is varying
from 0% to 3.1%, which is in accordance with the previously reported
value.