A genetically modified mouse gives us more insight on how sprinters and endurance runners differ. Looks more and more like a genetically caused protein deficiency may be one the things which makes “true” hardgainers.
http://hmg.oxfordjournals.org/cgi/content/abstract/ddm380v1
Commentary, including a note from one of the authors, here: http://scienceblogs.com/gnxp/2007/09/white_men_cant_run.php