Are novolin (insulin) and novolog interchangeable?

Different categories: Novolin is the brand name given to all human insulins made by novo nordisk. Under this brand are specific types of Insulin such as regular, nph, and 70/30. Novolog Insulin is a synthetic recombinant Insulin that is very rapid acting. I often get this question because NovoLog is more expensive than any of the novolins. However, they cannot be substituted for each other.