mutant.p53.df<-data.frame(
     mutation=as.factor(
        rep(c("GC->TA","AT->GC","GC->AT","GC->CG","AT->TA"),rep(3,5))),
     cancer=as.factor(rep(c("SCLC","NSCLC","Colon"),5),
     count=c(6,16,0,4,0,3,1,5,14,1,4,1,0,2,1))

 levels(mutant.p53.df$mutation)<-
           c("GC->TA","AT->GC","GC->AT","GC->CG","AT->TA")
 levels(mutant.p53.df$cancer)  <-c("SCLC","NSCLC","Colon")

 mutant.tab<-crosstabs(count~mutation+cancer,data=mutant.p53.df)

 crosstabs(count ~ mutation + cancer, data = mutant.p53.df)


 mutation|cancer
        |SCLC   |NSCLC  |Colon  |RowTotl|
 -------+-------+-------+-------+-------+
 GC->TA | 3     | 0     | 4     |7      |
 -------+-------+-------+-------+-------+
 AT->GC | 1     | 2     | 0     |3      |
 -------+-------+-------+-------+-------+
 GC->AT |14     | 5     | 1     |20     |
 -------+-------+-------+-------+-------+
 GC->CG | 1     | 4     | 1     |6      |
 -------+-------+-------+-------+-------+
 AT->TA | 0     |16     | 6     |22     |
 -------+-------+-------+-------+-------+
 ColTotl|19     |27     |12     |58     |
 -------+-------+-------+-------+-------+
 Test for independence of all factors
   Chi^2 = 33.64526 d.f.= 8 (p=4.710024e-05)
   Yates' correction not used
   Some expected values are less than 5, don't trust stated p-value