(CNN) -- Bradley Manning, the U.S. soldier recently sentenced to 35 years in prison for leaking classified documents, has announced that he would like to live as a female named Chelsea.
"As I transition into this next phase of my life, I want everyone to know the real me," Manning said in a statement read on NBC's "Today" show on Thursday. "I am Chelsea Manning. I am a female. Given the way that I feel, and have felt since childhood, I want to begin hormone therapy as soon as possible. I hope that you will support me in this transition."
As acceptance for gay rights grows in the United States, some activists say the "T" in LGBT has been left out. Confusion occurs over what "transgender" means, and how it differs from gay, lesbian or bisexual.
"Transgender is an umbrella term for persons whose gender identity, gender expression, or behavior does not conform to that typically associated with the sex to which they were assigned at birth," according to the American Psychological Association. "Gender identity refers to a person's internal sense of being male, female, or something else."
Some say the roots of gender identity issues are cultural -- that how a culture views a "boy" or "girl" and what they should or should not do contributes to gender issues. Others believe being transgender is a choice or a psychological problem. Some experts have hypothesized that exposure to hormones during pregnancy can lead to a baby's transgender identity since early research has shown androgens can affect fetal brain development.
Biology could also play a role, according to the National Center for Transgender Equality.
"A person (who) has XX chromosomes is generally considered female, while a person with XY chromosomes is generally considered male," the NCTE website says. "However, there are also people who have XXY, XYY, and other variations of chromosomes."
(Scientists aren't sure how common this is; the National Institutes of Health estimates that one in about 500 newborn males has an extra X chromosome.)
As more details about Manning's new identity emerge, here's what we know -- and what we don't -- about gender identity.
Sexual identity is different from gender identity.
"Sexual orientation is about who you are attracted to and who you fall in love with," explained psychotherapist Jean Malpas, director of the Gender and Family Project at the New York-based Ackerman Institute for the Family. "Gender expression and identity refer to the gender you feel comfortable expressing and identifying with, which might or might not be aligned with the biological sex you were assigned at birth."
Not all transgender people transition.
Some people who identify as transgender feel they were born into the wrong body. Not all transgender individuals want to undergo a physical transformation, such as a sex change operation, but some seek out such procedures.
Hormone therapies, which include estrogen or testosterone to alter the body's sexual chemistry, are one way that some transgender individuals choose to transition to a different sex. Estrogen therapies can soften the skin, reduce unwanted body hair and redistribute body fat to create a more feminine appearance, according to the National Center for Transgender Equality. Testosterone therapies lower a person's voice, increase body hair growth and end the menstrual cycle.
Surgery is another possibility. Plastic surgery, such as genital reconstructive surgery to create a penis or vagina, breast removal/augmentation or facial reconstruction surgery, can help transgender individuals make the transition they seek. Transmen -- women who want to become men -- can also have surgery to remove their ovaries or uteri. Transwomen can have surgery to reduce their Adam's apples.
We don't have good data on this population.
Robust studies about the transgender population are rare. And the percentage of Americans who see themselves as transgender is difficult to track, as many are not public about their identities. A 2011 study from the University of California, using information from four national and two state-level surveys, found about 700,000 American adults, or 0.3% of the population, identify as transgender.
It's no longer an official mental disorder.
The term "gender identity disorder" was once a diagnosis listed in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, a book of classifications of mental disorders widely used by American mental health professionals.
That old diagnosis meant that a man who believed he was destined to be a woman was considered mentally ill.
The most recent edition of this manual, the DSM-5, eliminates the term "gender identity disorder," which was long considered stigmatizing by mental health specialists and LGBT activists.
The new DSM refers only to "gender dysphoria," which focuses the attention on only those who feel distressed by gender identity issues. Having this still available as a diagnosis ensures a transgender person can still access related health care if needed. Hormone treatment would be one example. Another would be counseling for those who need help dealing with their emotions.
Nonconformity is common in kids.
Children as young as 3 can show early signs of gender identity issues, some mental health experts say. These children are not intersex -- they do not have a physical disorder or malformation of their sexual organs. The gender issue exists in the brain, though whether it's psychological or physiological is hotly debated.
Some children engage in gender nonconformity behavior, meaning they tend to take on roles and activities associated with the opposite gender. This could be a boy who grows his hair long or paints his nails, or a girl who wears only male clothing. From 2% to 5% of boys and 15% to 16% of girls exhibit gender nonconforming behavior, according to a 2012 Pediatrics editorial. For most, the behavior fades as they grow older.
This is not the same as a transgender identity, where the child is certain he/she was meant to be the opposite sex.
Transgender hormone therapy for children is a relatively new practice in the United States. Programs for transgender children exist in cities including Los Angeles, Seattle and San Francisco. The kids are treated by pediatric endocrinologists after long evaluations by mental health professionals.
CNN's Elizabeth Landau, Madison Park and Moni Basu and CNN contributor Ian Kerner contributed to this story.