R help - Sep 2011 - read.xlsx handle NAs

Hello,

I import a xlsx-table with read.xlx which contains NAs in 4 columns. In the
excel table these 4 columns contain floats/integers and the word NA. But when I
am importing the NAs are recognized as strings rather than as missing values.

How can I set that if a cell contains the word NA that this is a real missing
value? Is there any option (probably colClasses) where I can set it for the
import, and how should it look like? Or do I have to treat the imported
dataframe, and if yes how?

Thank you
Johannes
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