We consider  a marking procedure  of the vertices  of a tree  where each
vertex is marked  independently from the others with  a probability that
depends only on  its out-degree. We prove that  a critical Galton-Watson
tree conditioned on  having a large number of  marked vertices converges
in distribution to  the associated size-biased tree. We  then apply this
result to  give the  limit in distribution  of a  critical Galton-Watson
tree conditioned on having a large number of protected nodes.