redirection problem

[xiaawan]

Is signin a keyword in .htaccess file ? because I am having problem using the following rule.

RewriteRule signin http://www.mysite.com/index.php?cmd=s uperUser

but it works with following url

RewriteRule signIn http://www.mysite.com/index.php?cmd=s uperUser

having (I) capital in (signIn).
Please help me.