The RadEditor for MOSS (v.5.x) is based on the ASP.NET AJAX RadEditor. The base is the same, but the Sharepoint version of RadEditor is customized so that to be easily integrated with Sharepoint. Note that, the prior versions of RadEditor for MOSS (before 5.x) are based on the ASP.NET version of RadEditor.
More information on this topic can be found in the RadEditor for MOSS online help:
I hope this helps.
the Telerik team
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