Can you (1031) exchange partial proceeds from a sale?

[Ari Newman]

I'm not familiar with all the exchange guidelines.  For example, if I were to sell a property and net $50,000 profit, could I elect to exchange $40K of the proceeds towards a new purchase, pay short-term cap gains on $10K and keep the balance?  Or are you required to exchange ALL the net proceeds from the sale?

thanks