Can't find X11 headers in CentOS 6.4

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http://www.linuxquestions.org – I have recently upgraded to new computer and CentOS 6.4 I was using CentOS 5.1 and compiling C++ programs that required X11R6 header files and libraries. I can't find these in 6.4 although I've looked all over the place. It looks like X11 is installed (it come with the distribution right?) but the gcc compiler says it can't find files like X11/Xlib.h, X11/Xutil.h, X11/cursorfont.h, etc. 'locate' returned nothing for these files and all other X11 headers. Do I need further installation? Please help. (HowTos)