RB1 and growth hormone-producing pituitary gland neoplasm: For example, loss of pRb expression is tumor subtype-specific and inversely related to p16 expression: 31% of somatotropinomas and 17% of nonfunctional PAs failed to express pRb, whereas 25% of somatotropinomas and 66% of nonfunctional PAs showed altered p16 expression [28,36] On the other hand, this discrimination between inactivating various pathways applies only to those with similar downstream effectors.