[OS X TeX] \subject in letter

[Mohinish]

Hi,
I saw on a couple of sites that \subject was possible using LaTeX2e, in 
documentclass{letter}; however I get

! Undefined control sequence.
l.8 \subject
	{Motivation letter}

I saw from earlier posts that letter should satisfy most (simple) 
letter needs; and at this stage simple is what Im looking for; so would 
just like to know if its easy to insert a <subject> just before 
\opening{}; or this requires something (even marginally) complicated, 
in which case I'll just format by hand for now.

Any help is much appreciated!
-Mohinish

The code is:
-------------------
\documentclass[a4paper]{letter}
\address{Cognitive Neuro \\ Italy}
\signature{Mohinish}
\begin{document}
\begin{letter}{Summer Institute\\ USA}
\subject{Motivation letter}
\opening{To whom it may concern}
I have been working hard all summer.
\closing{Yours sincerely}
\end{letter}
\end{document}

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