Introduction

Cervical cancer and pre-cancer form a disease continuum ranging from cervical intraepithelial neoplasia (CIN) through microinvasion to invasive carcinoma; about 70% of the tumours are squamous and 30% are adeno- and adenosquamous carcinomas (Buckley & Fox, 1989). Most tumours are thought to develop from an area of intraepithelial neoplasia within the transformation zone (Coppelston & Reid, 1967). Cervical cancer is estimated to be the second most common female cancer with approximately 500 000 new cases per annum worldwide (Parkin, Laara & Muir, 1980). Sexually transmitted infections are recognized as one of the major risk factors and the active agents are thought to be specific types of human papillomavirus (HPV) (Munoz et al., 1992).

Papillomaviruses are small DNA viruses associated with benign and malignant proliferative lesions of cutaneous epithelium. Over 60 different types of papillomavirus have been described and they can be segregated into groups distinguished by DNA sequence homology and the specific lesions with which they are associated (de Villiers, 1989). HPV 6 and 11 are found most commonly in cervical condyloma, benign lesions that tend to regress spontaneously, and low-grade CIN. HPV 16 and 18 are the types most commonly associated with high-grade CIN lesions and invasive carcinoma of the cervix. The viruses will replicate only in specific differentiation stages of epithelia, which limits the use of in vitro culture methods for producing HPV. To circumvent this, molecular biological techniques have been utilized extensively to characterize HPV.