  We consider a proper holomorphic map form D to G domains in C^n and show that
it induces a unitary isomorphism between the Bergman space A^2(G) and some
subspace of A^2(D). Using this isomorphism we construct orthogonal projection
onto that subspace and we derive Bells transformation formula for the Bergman
kernel under proper holomorphic mappings. As a consequence of the formula we
get that the tetrablock is not a Lu Qi-Keng domain.
