simbg<-function(bg,L){
#bg is a vector of probabilities for A, C, G, T (1x4)
#L = length of sites to be simulated
seq<-rep(0,L)
dna<-c(1,2,3,4) #Numeric codes for DNA
seq<-sample(dna,L,replace=T,p=bg)
return(seq)
}