What Do We Now Know About the Early Developmental Milestones in Kleefstra Syndrome? A New Study of 100 People

When does a child with Kleefstra syndrome typically start to sit? When do they take their first independent steps? When might the first words appear? These are questions that concern almost every family after the diagnosis, but until recently, the answers were based primarily on small patient groups, isolated clinical experiences, and general descriptions of developmental delay.

A new study by researchers from Boston Children’s Hospital and Harvard Medical School comes to add more specific data on the early development of children with Kleefstra syndrome. The study entitled The Infant and Toddler Developmental Profile of Kleefstra Syndrome, with Shanna L. Yue as first author and Siddharth Srivastava as last author, was published online on February 6, 2026, in American Journal of Medical Genetics Part A and was included in the June 2026 issue.

Its value lies primarily in the size of the team: the researchers examined data from 100 individuals with molecularly confirmed Kleefstra syndrome, attempting to describe with numbers something that families have known experientially for years, that development in Kleefstra often follows a different pace, and also that there is significant variation from person to person.

What exactly did the researchers study

The study was retrospective review of medical records. This means that the researchers did not follow 100 infants from birth onwards. Instead, they retrospectively examined available information for 100 individuals with genetically confirmed Kleefstra syndrome and recorded the age at which specific early childhood motor and language milestones had been achieved. The median age of the participants at the time of recording was 9 years, and the group included 43 boys/males and 57 girls/females.

The researchers focused on four key milestones: body rolling (rolling), independent sitting, independent walking, and the appearance of the first words. At the same time, in a smaller subgroup of 24 participants, results were available from the Capute Scales, a developmental tool that separately examines language and visuomotor/cognitive skills in the early years of life.

When were the first milestones achieved?;

The results provide, for the first time, a fairly clear numerical picture of early development in a large clinical cohort with Kleefstra.

Η median age achievement of the key milestones was:

• body rotationrolling): 6 months
• independent seating: 10 months
• independent walking: 1.96 years, meaning approximately 23–24 months
• first words: 24 months.

The word «median» has great significance here. These numbers these are not deadlines that a child must meet, nor do they predict the trajectory of any specific child. The median value lies in the middle of the distribution: some participants acquired a skill earlier and others later.

Therefore, the 24 months for independent walking or the first words should not be used as a «normal limit» for Kleefstra. The important message of the study is that, at the group level, developmental delay is already evident from the early years and requires systematic monitoring.

What the Capute Scales showed

Of particular interest is the subgroup of 24 children for whom there was an evaluation with the Capute Scales. These scales examine two main areas: visuospatial and non-verbal problem-solving skills, as well as language development, which includes both comprehension and expression.

Researchers calculated developmental indicators, or Developmental Quotients (DQs). It is important to clarify that a DQ It is not IQ and should not be read as a measure of a child's future intelligence or potential. It is a way of capturing the level of development of a specific skill in relation to chronological age at the time of evaluation. The Capute Scales are used precisely for the quantitative assessment of linguistic and visuomotor/cognitive skills in early childhood.

In the study, the median developmental indices were:

• visuomotor skills: IQ 53, with an interquartile range of 42–71
• overall language development: DQ 56, range 42–67
• expressive language: DQ 52, range 35–60
• language understanding: IQ 50, range 42–61.

These values confirm that developmental burden in early childhood does not exclusively concern speech or mobility. It can affect multiple areas simultaneously, though with a different degree of impact on each child.

At the same time, the fairly wide price ranges indicate something equally important: there is no single “developmental profile” that describes every child with Kleefstra. Even within this small subgroup, there was notable variability.

«First words» do not describe the whole communication

The finding that the median age of onset of the first words was approximately 24 months also requires careful interpretation.

The appearance of a first word does not mean that subsequent language development will follow a specific course. In Kleefstra syndrome, we now know that the spectrum of speech, language, and non-verbal communication is particularly wide.

A previous study of 103 individuals with Kleefstra showed great variability in language and cognitive function, while speech disorders were very common even in verbal individuals. In the same work, alternative and augmentative communication means, such as sign language and speech-generating devices, were important for 61 of the 103 individuals studied.

For this reason, monitoring a child should not be limited to the question «did they speak or didn't they speak?». It needs to be examined as a whole how does it communicatewhat he/she understands, how he/she expresses needs and choices, whether he/she uses gestures or signs, how he/she interacts socially, and whether he/she needs support with alternative means of communication.

From landmarks to developmental pathways

Perhaps this is also the most important contribution of the new work.

In rare neurodevelopmental disorders, it is not enough to know that «there is a developmental delay.» To better organize care, and in the future to evaluate new therapeutic approaches, there is a need quantitative data on the natural course of development.

We need to know when specific skills typically appear, how large the variability is among children, which developmental domains are most affected, and how they change with age.

The work by Yue and colleagues does not yet answer all these questions. However, it establishes an important initial benchmark for the early years of life in Kleefstra syndrome. And it shifts the discussion from the general phrasing «children with Kleefstra have developmental delay» to something much more useful: an attempt to describe their actual developmental trajectory with measurable data.

For families, the message is not that a child «must» sit at 10 months, walk at two years, or say their first words at 24 months. The message is that development needs to be monitored systematically, individually, and early, with access to appropriate support services and without the average numbers of a study being turned into a prediction for the future of a specific child.