I'm reading the paper called: Differential expression analysis for sequence count data. in the part of model description they have mentioned that: "We assume that the number of reads in sample j that are assigned to gene i can be modeled by a negative binomial (NB) distribution,".
I don't understand why it can be modeled by a negative binomial?
Intuitively, negative binomial distribution is the probability distribution of independent trails for k successes.
Would someone elaborate more that, why their assumption are make sense?