The fourth paper published in this series, entitled "The Relationship of Epinephrine and Glucagon to Liver Phosphorylase: IV Effect of Epinephrine and Glucagon on the Reactivation of Phosphorylase in Liver Homogenates", came out in 1956 and was the most influential and groundbreaking of those released. In this paper, Sutherland and associates furthered their investigation of epinephrine and glucagon. The key to the success of this experiment was the use a homogenate of liver cells rather than intact liver cells, as they had been doing in their previous experiments. The general consensus among researchers at that time was that the study of hormones was only possible using intact cells; this was the first instance that a hormone pathway was studied using a cell homogenate. Sutherland and his co-authors were able to observe similar effects in liver homogenate to what was observed in whole liver slices. More importantly, they were able to observe this response in two stages. This stage response was characterized by the particulate fraction producing an unknown heat stable factor in the presence of the hormones epinephrine and glucagon. This factor then stimulates the formation of liver phosphorylase in a fraction of the homogenate where the hormones are not present. This unknown heat stable factor, which was produced in the presence of hormones and ultimately led to the secondary formation of liver phosphorylase, was later termed cyclic AMP.