At first glance, the protected classes sound simple enough.  The federal Fair Housing Act (FHA) lists seven:  race, color, religion, national origin, sex (gender), disability, and familial status (presence of children under age 18 in the household or a pregnant woman).  Some state, city, and county anti-discrimination laws include additional protected classes. 

In response to a client request, we took a look around at the state of the law involving another common protected class which is not in the FHA:  source of income.  Our research found that at least thirteen (13) states and the District of Columbia include source of income as a protected class. Furthermore, numerous cities and counties have identified source of income as a protected class.  We started counting and identified a total of at least forty-three (43) geographic areas that have enacted source of income protection to their anti-housing discrimination laws.  
This continues to demonstrate what I have previously noted:  source of income remains an issue that will increase over time as more jurisdictions include it as a protected class.  This is of particular concern to those apartment owner/managers with properties located in various states (or even multiple cities/counties in a single state).  Each management office certainly needs to know the laws of the jurisdiction it is in and our staff should be prepared to answer the source of income question.  It gets more complicated if the apartment owner/manager has a centralized call center dedicated to responding to inquires for various properties located in a number of areas.  In a perfect world, the call center needs to be able to access the law in the various state, cities, and counties.  
If you or your leasing office staff get a call about source of income and are uncertain how to respond, take a message and contact competent counsel.  I have seen any number of fair housing cases filed because of either a misunderstanding or just not being aware of how to respond to this important question.   And then you really need to spend time with someone like me.
Just A Thought.