THE twilight zone of the genders, that grey area between masculinity and femininity, is swelling and those who fall into the shadowy chasm are desperate to clamber out - to become ''proper'' men or women, at any cost. Doctors estimate up to 40 Hong Kong people have changed their sex after years of battling stigma, taunts, mental examinations, hormone therapy and painful surgery. Another 30 hopefuls are enrolled in the government gender-change programme, which guides successful applicants down the rocky road to changing their sex. Whether bar girls, civil servants, doctors, managers, shop workers, labourers, hairdressers, designers, clerks, even a Chinese herbalist - they run the gamut of Hong Kong society. Most have turned to the government-funded Gender Identity Team (GIT), based at Queen Mary Hospital. But an unknown number also approach private doctors. A leading sex-change surgeon with the GIT, Dr Chow, also works privately. He cannot reveal his full name in accordance with medical advertising rules. ''I can't tell [exactly] how many transexual operations I have done,'' he said. ''I guess, around 20. I don't see any reason why the number of transexual operations should decrease in the future. It might increase because more and more people are aware of the service.'' Many Hong Kong transexuals are men who become women. Most are in their mid-20s, but one 16-year-old boy has asked to become a woman and a Hong Kong civil servant had his sex changed at the age of 53. Dr Chow estimates that 60 per cent of male-to-female patients work as prostitutes or hostesses in nightclubs and topless bars around Hong Kong. ''Some are already working in that field before they have the transexual operation,'' he said. ''They can have their breasts augmented just like any female. Some people question why we are making prostitutes, but prostitutes are still patients. Why should we reject a prostitute who wants a transexual operation?'' To outsiders, the men appear to be women even before the operation, dressing in skirts and blouses and working in offices, bars and shops. Women who yearn to be men do the same; dressing, acting and in every way considering themselves male. ''One of the criteria to select them is that they must dress, work and behave as the opposite sex for at least half a year, but usually they've dressed as the opposite sex since age 16 or 17,'' a government specialist said. ''The surgery is just the lastpart. It gives them the satisfaction, but they feel they are the opposite sex already.'' GIT consultant surgeon, Dr Albert Yuen Wai-cheung, said one of his most recent female-to-male patients, a Chinese herbalist, had relished the thought of breast-removal. ''He's a very big man,'' Dr Yuen said. ''People called him 'fat boy' and teased him about his breasts.'' Those intent on a sex-change operation are usually channelled to psychiatrist Dr Ng Man-lun, GIT co-ordinator. Patients, only those above 21, are assessed by Dr Ng and a clinical psychologist. ''Some of them may not be transexuals,'' Dr Ng said. ''They may just have an ambiguity in their genetic makeup or other conditions which can make them want to become a member of the opposite sex.'' The GIT has operated on 22 patients and rejected only ''four or five'' applicants on grounds of mental illness or instability. ''People don't come for this kind of operation for fun,'' Dr Ng said. ''We refer them to a social worker to determine how long they've been leading the life of the opposite sex and how well they've been accepted by their family and friends.'' Once accepted, the patient begins hormone therapy. Female hormones soften the features and suppress libido, while male hormones induce facial hair. ''Some operations cannot be done perfectly due to various reasons, . . . the type of operation or the surgeon's skills,'' Dr Ng said. ''Pain is very common, but usually they accept it well.'' Dr Yuen performed about six operations at Ruttonjee Hospital last year, producing equal numbers of men and women. ''Patients have to be very definite, because it's irreversible,'' he said. ''Once it's done, you cannot have regrets.'' A male-to-female transexual will spend a full day - at least 12 hours - under the scalpel and a week in hospital to recover. Under Hong Kong's public health system, the patient pays $43 a day. Privately, the package costs $60,000 to $100,000. Surgeons use silicone to build breasts; then create a vaginal tunnel, and line it with the skin of the amputated penis. A female-to-male operation is more complex, involving two full-day operations and a month in hospital. ''The first step is to remove the breasts, the vagina and ovaries,'' Dr Yuen said. ''The second is construction of the penis. This can be dangerous. It involves taking a skin flap from the forearm, plus blood vessels and joining it to the urethra. If the joining of the blood vessels fails, the whole organ can die.'' Doctors try to keep tabs on patients for up to a year, often providing hormone injections, but a number disappear into the community. Dr Yuen said male-to-female transexuals enjoyed full sex lives, but none of Hong Kong's female-to-male transexuals had asked for the complex operation to give them a similar ability. ''After surgery they are happier, more confident, more comfortable with their jobs,'' he said. ''They're more comfortable in their dressing, they're more free. ''Some of the male-to-females look very beautiful and some are average. Similarly, some of the female-to-males are very strong with big builds; you wouldn't believe they had been female.'' One 22-year-old hairdresser returned to Ruttonjee after a recent male-to-female sex change. ''She's very attractive after the surgery,'' Dr Yuen said. ''One day she came up in the lift and everyone was following her, saying she was very attractive.'' The GIT's legal adviser is solicitor and medical doctor So Kai-ming, who tells patients of the legal tangles which arise from a sex change. Transexuals can change the sex listed on their Hong Kong ID cards and passports, subject to the discretion of the Immigration Department. Officials say the two-month process is simple - the transexual completes an ordinary amendment form and produces a medical certificate. ''But whatever the Immigration Department does won't change your legal status,'' Dr So said. ''It's an administrative procedure for your convenience. Your legal status depends on your chromosome makeup. These people, no matter what surgery they've gone through, still have the same chromosome makeup.'' ATRANSEXUAL female can charge an attacker with sexual assault, but not rape, because ''there cannot be rape between two men'', and transexuals face a dilemma when using public toilets. ''A person cannot go to the toilet of the opposite sex,'' Dr So said. ''Every time they go to a public toilet they are committing an offence - of course, nobody knows about it.'' But perhaps the biggest problem facing these people is marriage. ''Whatever ceremony you go through, legally you will never be married,'' Dr So said. ''In law, your cohabitation [between a woman and a transexual male] will still be between two women. ''One obvious case was when a transexual male married a female. When the couple applied for right of abode in Britain, his wife got it and he didn't because, legally, they weren't married. Usually, under the scheme, when one partner has right of abode the other gets it, too.'' Doctors are still divided over why some adolescents develop an unwavering certainty they were meant to be a different gender. Dr Ng said he did not know of a transexual regretting the decision. ''The most popular theory is that it's a congenital abnormality,'' Dr So said. ''What causes this abnormality is still under investigation. ''It's probably due to hormonal imbalances of the mother during pregnancy. The foetus may develop with the body of a man but the mind of a woman, or vice versa.''