Hormone Replacement Therapy (HRT) and Biological Changes
Hormone Replacement Therapy (HRT) is a significant aspect of the medical transition for many trans women, leading to profound changes in their biology. HRT affects various biological aspects, including skin texture, fat distribution, breast development, bone density, and even brain morphology. These changes are not just superficial but occur at the molecular and cellular levels, aligning trans women's biochemistry more closely with that of cis women. As one user explains, "After enough time on HRT we have more things biochemically similar between us and cis women than us and cis men." – threefriend source [citation:4f1b2825-a124-407a-8f34-d1b3865dd0e6].

The Role of Gene Expression
Gene expression plays a crucial role in the biological transition of trans women. HRT can influence which genes are expressed, leading to changes in the body's physical characteristics. This process can result in the development of female organs and other secondary sex characteristics. One user highlights the potential of gene expression, stating, "Reproduction is even possible since science already achieved male skin cells to female gametes and disabling the mis protein... produces uterus in 'male' stem cells." – Butteromelette source [citation:231b49be-d4a8-44fb-b951-041999734a97].

Neurological and Phenotypic Alignment
Trans women often describe a sense of alignment between their neurological identity and their physical bodies after undergoing HRT. This alignment is not just about external changes but also involves internal, neurological aspects. As one user notes, "They also don't realize that brains are a major component of biology and the fact that we feel dysphoria due to the disconnect between brain and body kind of proves our brains are female." – Forsakened_Bia source [citation:6530a69e-762c-4ca6-9c17-bb0a1004b51c].

Challenging the Chromosome Argument
The argument that chromosomes are the sole determinant of biological sex is challenged by many trans women and researchers. They emphasize that biological sex is not binary and involves more than just chromosomes. Hormones, secondary sex characteristics, and brain structure all play significant roles. As one user explains, "Biological sex is not binary, and it’s not determined purely by chromosomes... these are all influenced by our medical transition." – ImmediateDamage1 source [citation:5d14f93b-87a7-4187-a263-c38cd932c2d9].

Conclusion
The journey of transitioning for trans women involves significant biological changes that go beyond superficial appearances. Through HRT and the influence of gene expression, trans women can experience profound changes in their bodies that align with their gender identity. These changes challenge traditional notions of biological sex, emphasizing that it is not solely determined by chromosomes. This journey is deeply personal and transformative, offering hope and alignment for those questioning their gender identity.