In addition to somatotroph tumors, other subtypes of GH- and PIT-1-positive tumors cause acromegaly: mammosomatotroph tumors and mature plurihormonal tumors (characterized by the expression of GH along with additional hormones such as prolactin (PRL) and thyrotropic hormone), mixed somatolactotroph tumors (comprising two distinct populations of somatotroph and lactotroph tumor cells), and poorly differentiated PIT-1–positive tumors [2, 3]. The gene discussed is POU1F1; the disease is acromegaly.