The envelope proteins induce cell transformation leading to ovine pulmonary adenocarcinoma (OPA), a contagious lung cancer of sheep and goat. They bind to the HYAL2 receptor for cell entry. Env proteins probably do not act as oncogenes by themselves, but may rather liberate an oncogenic factor that would normally be negatively regulated. One mechanism of transformation seems to involve activation of the phosphoinositide-3-OH kinase (PI3K)/Akt pathway but does not involve the virus receptor HYAL2, and the other seems to involve Env binding to HYAL2, HYAL2 degradation, and activation of the MST1R receptor tyrosine kinase, which is normally suppressed by HYAL2