The only wires needed in a mouse cable are as follows: TxD and RxD for data transfer, RTS and/or DTR for power sources, and ground. Translated into pin numbers, they are:
9-pin port 25-pin port
TxD 3 2
RxD 2 3
RTS 7 4
DTR 4 20
Gnd 5 7
The above table may be of use if you wish to make adaptors between 9- and 25-pin plugs, or extension cables.