Nonexistence of regular maps between homogeneous projective varieties

We study non-existence of non-constant regular maps from a partial flag variety $X=G/P$ to another partial flag variety $X'=G'/P'$ and prove that there does not exist any non-constant regular map from any partial non-complete flag variety $X$ to any complete flag variety $X'=G'/B'$. We also formulate a general conjecture on the non-existence of non-constant regular maps from $X\to X'$.