[R] select on letter

[naomi.duijvesteijn at ipg.nl]

Hi,

I was wondering if anybody can help me. In the small data set below I would like to select the index which doesn't contain the letter 'N' in the variable 'letters'. How can I discard these rows when the letter has a different position everytime (but the same letter for the whole column)?

index<-c(1,2,3)
letters<-c("CCTTGGAA", "NNTTGGAAT", "AACCTTNN")
z<-data.frame(index,letters)


 index   letters
1     1  CCTTGGAA
2     2 NNTTGGAAT
3     3  AACCTTNN


Thanks in advance!
Naomi