Asus M3N72-D - Manual (Page 62)

2-38 Chapter 2: Hardware information
10. Optical drive audio connector (4-pin CD)
These connectors allow you to receive stereo audio input from sound sources
such as a CD-ROM, TV tuner, or MPEG card.
11. Serial port connector (10-1 pin COM1)
This connector is for a serial (COM) port. Connect the serial port module
cable to this connector, then install the module to a slot opening at the back
of the system chassis.
The COM module is purchased separately.