How to look inside of "as" methods in R. To convert one S4 object into another we define "setAs" function and use is as "as". setAs returns a "coerce<-" method. So to look up what is going on one have to look inside of "coerce" For example coercion from MSnSet object into Expression set. getMethod("coerce", signature=c("MSnSet","ExpressionSet")) This returns the code of what exactly is going on during the conversion.