A little boy who decided he was a girl trapped in a boy's body has become one of the youngest-ever children to have his decision backed by the NHS - aged just four.
With his blonde pigtails and purple tutu, Zach Avery, now five, has been living as a girl for more than a year.
Little Zach was just three when he began refusing to live as a boy, instead choosing to wear pink dresses and ribbons in his long, blonde hair - because he has Gender Identity Disorder (GID).
Mum Theresa Avery, 32, said her son used to be a "normal" little boy who loved Thomas the Tank Engine, but suddenly at the end of 2010, he decided he wanted to live as a girl.
He became obsessed with the children's TV character Dora the Explorer and started dressing in female clothing.
Theresa and Zach's father Darren, 41, became worried by their son's behaviour and took him to the doctor.
He was officially diagnosed with GID by NHS specialists at the Tavistock and Patman Foundation Trust in London, making Zach one of the youngest affected children in the UK.
Mum-of-four Theresa said: "He just turned round to me one day when he was three and said: 'Mummy, I'm a girl'. I assumed he was just going through a phase and just left it at that.
"But then it got serious and he would become upset if anyone referred to him as a boy.
"He used to cry and try to cut off his willy out of frustration."
Specialists explained to Theresa and Darren that gender identity disorder is a conflict between a person's actual physical gender and the gender that person identifies himself or herself as.
Theresa said: "They told us that although he had a male body, his brain was telling him he was a girl."
And Zach's school - Purfleet Primary in Essex - has even turned their toilet block gender-neutral to support him.
Theresa added: "They have changed the toilets for Key Stage 1 pupils into Unisex instead of male/female and they address him as a girl, which is what he wants.
"When he gets a bit older, to Key Stage 2, then obviously the law changes and there will be more difficulties surrounding the bathroom issue, but we'll cross that bridge when we come to it - it may be that Zach will use the staff toilets.
"We explained to the other kids at the school that Zachy's body was that of a boy but in his brain he was a girl. We said Zach was just happier being a girl than a boy.
"But the other kids haven't batted an eyelid, they've accepted Zach as Zach and there's been no problems at the school with bullying.
"The school has been brilliant and really, really supportive."
When he goes to school, Zach, known affectionately as Zachy, wears a girl's trouser uniform and black boots with pink trim, which his mother said is female but still neutral.
And mum said that although she misses her little boy, the family is very supportive of Zachy.
She said: "He just wants to be like a little girl and he's very happy with his long blonde hair, pink and red bedroom and a wardrobe full of girls clothes.
"He likes playing with his sister's old toys but he still loves Dr Who too and playing with his brother. And we still put some neutral clothes in his wardrobe if he ever decides he wants to wear them.
"We leave it up to him to decide what he wants to do - if he changes his mind and wants to be a boy again then he does, but if he doesn't, he doesn't.
She admitted: "I would love to have my son back, but I want him to be happy. If this is the route he wants to take - if this is what makes him happy - then so be it. I would rather him have my full support.
"People need to be aware of this condition because it's very common but even many family support workers have never heard of cases in children. There are people out there but they don't want to talk about it."
Figures from the Tavistock and Patman Foundation Trust clinic - the national body for GID - revealed 165 children have been diagnosed with GID this year.
A spokesperson at Tavistock Clinic in London said they were unable to comment on individual case, but only seven children under the age of 5 were diagnosed last year - making Zach one of the youngest.
The spokesperson said: "Tavistock Clinic had 97 referrals in 2009/2010; 139 in 2010/2011 and thus far this year 165 referrals.
"The trend in referrals has been up over the years - this may reflect greater awareness.
"We see children and young people up to the age of 18, from across the UK, who are experiencing difficulties in the development of their gender identity.
"This includes children who are unhappy with their biological sex. Some may be boys who prefer activities and role associated with the opposite sex, some may also identify as the opposite sex and vice versa for girls.
"In general when younger children are referred it is in relation to cross gender preferences in play, play mates and activities. It is more unusual for children of this age to express cross gender identification - that is the wish or belief that they belong to the opposite sex.
"The diagnosis of GID is made by the key workers working with the young person. We will also assess their general wellbeing. We remain in contact with young people often for many years.
"Our aim is not to predict or direct the outcome, but rather to support the young person in their general development as well as develop a trusting collaborative therapeutic relationship in which it is possible to openly explore their feelings about their gender."
FACTS ABOUT GENDER IDENTITY DISORDER
Gender Identity Disorder in children (GIDC) is the formal diagnosis used by psychologists and physicians to describe children who experience significant gender dysphoria.
Gender identity disorder is a conflict between a person's actual physical gender and the gender that person identifies himself or herself as. For example, a person identified as a boy may actually feel and act like a girl.
Males are diagnosed with GIDC 5-30 times more than females.
The person experiences significant discomfort with the biological sex they were born. Children with GID may feel disgusted by their own genitals, have anxiety or depression or believe they will grow up to be the opposite sex.
The exact cause of gender identity disorder is not known, but several theories exist, including one that it may be caused by generic (chromosomal) abnormalities.