P.O.Box?

In formal letter writing ,where should the “P.O.Box 1234” be written in the sender adress ?

If you send to a PO Box, it usually replaces the street name and number.

Thank you for your reply . Frankly speaking ,I got perplexed by different answers ! Some say it should be on the last line of the address ,others say it depends on the custom of the country and its culture while you say it usually replaces the street name and number!! I am sure you are right but what if the question provides the complete address of the sender including number of the house , name of the street and city?