about ajax in asp.net xml.open

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function getCities(obj){ var oXmlHttp = createXMLHttp();	 oXmlHttp.open("GET", "GetCities.aspx?StateInitials=" +obj.value, true); oXmlHttp.onreadystatechange = function()  {   	  if(oXmlHttp.readyState == 4)	{		if(oXmlHttp.status == 200)		{			displayMsg(oXmlHttp.responseText);		}else		{			displayMsg("An error occurred:" + oXmlHttp.statusText);		}	}} oXmlHttp.send("");	}

the page is not opening oXmlHttp.open("GET", "GetCities.aspx?StateInitials=" +obj.value, true);and there is no error in my javascript.. what could be the problem?