Microsoft.Exchange.Data.ObjectId Into String Variable

I have a value (Microsoft.Exchange.Data.ObjectId) that I'm trying to get into a variable and I'm not sure how.

This is what I have been trying:

PS> $USER1 = Get-MAILBOX USERX

PS> $USER1.Identity

IsDeleted                    : False
Rdn                              : CN=USERX
Parent                         : MYDOMAIN.NET/Exchange/Sites/Users
Depth                          : 7
DistinguishedName  :CN=USERX,OU=Users,OU=Sites,OU=Exchange,DC=MYDOMAIN,DC=NET
IsRelativeDn              : False
DomainId                   : MYDOMAIN.NET
ObjectGuid                : 25e15e62-3cf3-4705-a3ef-2b35b4ce631c
Name                         : USERX

When I type:

PS> $USER1 | FL Identity

I get:

Identity : MYDOMAIN.NET/Exchange/Sites/Users/USERX

How can I put Identity into a variable so I can get:

PS> $USRIDENTITY

MYDOMAIN.NET/Exchange/Sites/Users/USERX

Thanks!

Kevin