Deductions can be made in this order:  a6=B.  a1=d2=E.  f1=e2=either d3 or d6.  But if f1=e2=d3, then the remaining 3 instances of that symbol (whatever it is) must be in columns a-c rows 4-6, in which there are only 2 regions, which is impossible.  So f1=e2=d6=D.  Then if c3=D, there is no place for D iin the c4 region, so a3=D, b4=D, c5=D.  a2=C, b2=A, c2=B, c3=F, b3=C, a5=F, b6=E, c4=E, f5=E, b1=F, c6=A, d4=F, d5=C, e5=A, d3=B, e4=C, e6=F, etc.