By Duncan Kennedy
BBC News, Mexico City
Go to the centre of Mexico City after dark and you can see the women start to colonise the district.
The grey suits that are everywhere during daylight hours give way to multi-coloured miniskirts as the time arrives for a different kind of product to be traded.
There are not enough street corners to accommodate all the women. Instead, there are rows of them along the main boulevards.
The parade of women means the male clients can stay in the dry warmth of their cars as they make their impersonal choice.
One estimate says there are 3,000 prostitutes in the city at any one time.
But what happens when they get older and can no longer walk the streets?
One answer lies behind an unremarkable brown door of a two-storey block in the city's poorer northern neighbourhood.
Push open the door and you walk into what is believed to be the world's first retirement home for prostitutes.
A fountain in its central court yard gives it the tranquil feeling of a home for pensioners in Florida.
Here all of the 30 women who have so far moved in are former workers in the sex industry.
The house is the idea of Carmen Munoz, herself a prostitute for 20 years.
"It's taken me a long time to get this place opened," she says. "We had to convince the local government and the police it was needed. But when I saw elderly women lying in the streets with nowhere to go, I knew I had to act."
An enlightened city mayor and private donors are paying for the home, Carmen says.
As we talk, one woman shuffles by on a walking frame. Carmen turns to me and whispers: "She's 90 and spent 40 years as a working girl."
Set around a central square with walls painted in blue and yellow, the house is called Casa Xochiquetzal. It is named after the Aztec goddess of beauty and sexual love.
We go into the room of Maria, 75. On one wall hangs a small picture of Jesus. A collection of six black, brown and blue hand bags are pinned to another.
It is a sign, she tells me, of her determination to retain some femininity in a career of cold, anonymous, encounters.
After providing a lifetime of comfort for others she, like all those here, was condemned to a life of solitude herself.
"Before I came here, I lived with people who used to hurt me. If I couldn't pay the rent they used to make me suffer very much," Maria says, before breaking down in tears.
Later, we see other women and more rooms, most of them equipped with nothing but a bed.
Any money the women earned was long ago siphoned off by pimps and corrupt police officers.
I see a young boy and girl playing by the fountain. Carmen tells me there is an occasional visit by a grandchild. But the sons, daughters, brothers and sisters of the women stay away.
"In a country like Mexico which is very conservative and religious, those friends and family don't want to know," she says.
Martha, 74, has not seen her two sons for years. The house now provides the dignified sanctuary so long denied her.
"I have many comforts here," she says. "There's food and a roof over my head. I don't want much, just security and to be with friends."
Until now, Martha and the other residents were proof that the sex industry had a forgotten demographic: elderly, former practitioners, discarded by the cruel forces of a market that penalises the imperfections of old age.
But after a life of violence, discrimination and exploitation, these women have at last found people who are showing them compassion.
Prostitution is the oldest profession in the world, but some of its oldest professionals now have a place to call home.